Buoyed by U.S. and allied airstrikes that relieved a siege of Benghazi, Libyan rebels launched an offensive early Monday aimed at retaking the strategic city of Ajdabiya, as Western warplanes continued pounding forces loyal to longtime leader Moammar Gaddafi.

Explosions and plumes of smoke marked the scene of the strikes against Gaddafi’s forces in Ajdabiya, about 100 miles south of the rebels’ de facto capital of Benghazi, and advancing rebels cheered when Western warplanes flew overhead. The rebels by Monday had regained control of Zuwaytinah, an oil terminal about 16 miles northwest of Ajdabiya that had been captured by loyalist forces last week, news agencies reported

Speaking at a news conference Monday in Santiago, Chile, the latest stop on a South and Central American trip, President Obama defended his decision to deploy the U.S. military in Libya, saying the international community couldn’t “simply stand by with empty words” in the wake of continued attacks by the Libyan government on its rebels.

Despite the airstrikes, however, Gaddafi’s forces were digging in outside Ajdabiya, which straddles highways that go north to Benghazi and east across the desert to Tobruk.

From a point about five miles from the northern entrance to Ajdabiya, rebels jumped into dozens of vehicles and made a massive push toward the city Monday when they heard jets in the air and the sounds of bombardment. But after about half a mile, the rebels came under fire from loyalist tank and mortar shelling and promptly turned back.

Afterward, rebel commanders said they plan to wait for more allied airstrikes against Gaddafi’s forces before pushing forward again.

Some Gaddafi opponents were concerned that the coalition has primarily focused on Benghazi even though rebels are holding out against the government in other areas, notably the western city of Misurata, Libya’s third largest, where Gaddafi’s troops launched a major assault on the eve of the U.N. vote authorizing the use of force.

Gaddafi forces reportedly made further advances on Monday, with government tanks rolling into the city and attempting to fight their way to the symbolic central square. At least 14 people were killed and more than 100 were injured, said a doctor at the hospital in Misurata who spoke on condition of anonymity because he fears the consequences of being named.

He said rebel fighters managed to hold onto the square, but Libyan forces had secured control of several neighborhoods, and had positioned snipers on buildings on a main road leading through the city, where they were shooting at anyone who moved. Tanks and artillery continued to pound rebel held neighborhoods throughout the day. Overnight, a helicopter attacked the antenna of the local radio station.

“What no-fly zone?” he asked. “I am sure the U.N. has forgotten us.”

Libyan government spokesman Moussa Ibrahim said that Misurata had been “liberated”, but that pockets of what he called Islamic extremists who are prepared to die for their cause are continuing to resist government forces.

He said coalition forces had struck a harbor 27 kilometers west of Tripoli on Monday, and the airports in Gaddafi’s hometown of Sirte and in the southern town of Sabha in earlier strikes. He claimed all these targets were civilian facilities and that “many” civilians had been killed. But he had no update to the original number of 48 civilians killed in the first night of bombing issued by the government.

The Al Jazeera network reported that the rebel-held town of Zintan southwest of Tripoli had also come under heavy bombardment on Monday.

In Stuttgart, Germany, the commander of coalition forces involved in the Libya campaign, U.S. Army Gen. Carter F. Ham, said the aim now is to extend the U.N.-mandated no-fly zone southward and westward from Benghazi to cover other cities, including Misurata and Tripoli. In doing so, he said, “it is likely we will encounter the regime’s mobile air defense systems . . . and will certainly attack them.”

Ham said the allied mission was to “protect civilians from attack by the regime ground forces” and not to provide close air support for the rebels or “support opposition forces if they engage in offensive operations.” Obama reiterated that administration position during his news conference in Chile.

In a news conference with Pentagon reporters by video hookup, Ham, who heads the U.S. Africa Command, also said the allies are not targeting Gaddafi personally or seeking to destroy the Libyan armed forces. He said the opposition consists of civilians who are trying to protect their homes and families, as well as military forces with heavy weapons, and he acknowledged that it might be difficult to distinguish between the two in deciding when civilians are being attacked and thus subject to allied protection.

On Sunday, coalition aircraft — roughly half of them American — flew about 60 sorties, and there have been 70 or 80 sorties Friday, “well over half” by planes from other countries, Ham said. The result has been that Gaddafi’s forces “now possess little will or capability to resume offensive military operations,” he said.

Ham defended an attack on Gaddafi’s compound in Tripoli, saying the large complex included air-defense systems and a command-and-control facility, which was the main target of the strike. The attack degraded “the regime’s ability to control its military forces in the attack on civilians” and thus had a “very direct relationship” with the allies’ mission, he said.

He added: “I have no mission to attack [Gaddafi], and we are not doing so. We are not seeking his whereabouts or anything like that.”

In Benghazi, Libya’s second-largest city, anti-Gaddafi spokesmen said the rebels ultimately still plan to march on Tripoli, the Libyan capital where the 68-year-old strongman remains ensconced. Opposition spokesman Ahmed al-Hasi said the rebels would welcome more airstrikes but want to achieve their aims without the intervention of foreign ground troops, Reuters news agency reported.

New fighting also was reported Monday in the besieged city of Misurata, where rebels have been battling to hold onto their westernmost stronghold, Arabic language television networks said. Rebels claimed that the Gaddafi government was bringing civilians from nearby towns to serve as human shields for his forces laying siege to Misurata, Libya’s third-largest city about 130 miles east of Tripoli.

Gaddafi has threatened the West with a “long, drawn-out war,” but his forces have offered no serious military challenge to the establishment of a no-fly zone over his country.

In remarks broadcast on state television, Gaddafi also denounced foreign forces as “Nazis” and claimed that U.S. and European airstrikes have killed “thousands” of civilians. Allied officials denied that coalition strikes have caused significant civilian casualties.

“We judge these strikes to have been very effective in significantly degrading the regime’s air defense capability,” Vice Adm. William E. Gortney, director of the Pentagon’s Joint Staff, said Sunday.

If the military gains were clear, the international and domestic political support for the U.N.-authorized campaign seemed to weaken. The Arab League voiced concern about civilian deaths, and leading Republicans demanded clarity on the ultimate goals.

In Moscow, Russian Prime Minister Vladi­mir Putin on Monday condemned the U.N. Security Council resolution that authorized military intervention in Libya, calling it “defective and flawed” and saying it “resembles medieval calls for crusades.” The Security Council adopted the resolution by a 10-0 vote Thursday. Five members abstained, including Russia and China, which have veto power.

China on Monday stepped up its criticism of the U.S.-led airstrikes against Libya, using the Communist Party’s main media organs to say the military intervention undermines the United Nations charter.

"The military attacks on Libya are, following on from the Afghan and Iraq wars, the third time that some countries have launched armed action against sovereign countries," said a commentary in the Communist Party’s main newspaper, People’s Daily.

Obama has declared that Gaddafi “must leave,” but Adm. Mike Mullen, chairman of the Joint Chiefs of Staff and the administration’s most visible spokesman Sunday, acknowledged that the outcome of the conflict remains uncertain.

Despite a plume of smoke around one of Gaddafi’s compounds in Tripoli, U.S. officials said that they were not aiming to kill the Libyan leader.

“At this point I can guarantee he is not on the target list,” Gortney told reporters at the Pentagon. “We are not targeting his residence.”

Air Force B-2 stealth bombers as well as Marine Harrier jets flying from a ship in the Mediterranean followed up on the dozens of Tomahawk cruise-missile strikes that opened the assault on Libya, called Operation Odyssey Dawn. U.S. fighter jets mounted attacks on Libyan soldiers advancing on the rebel-held city of Benghazi as part of a broader mission to protect the besieged opposition forces from being overrun, said a senior U.S. military official.

The strikes, which were carried out by 15 U.S. fighter jets as well as French and British planes, left a smoking graveyard of military vehicles outside Benghazi.

“We thank the international community for their serious steps to kill this murderer,” said an opposition fighter in the rebel capital who gave his name as Adam al-Libi. “If God is willing, we will win.”

The United States also mounted strikes with satellite-guided bombs on an airfield outside the coastal city of Misurata, where the Libyan air force maintained fighter jets in hardened shelters. Gaddafi continued to keep the Soviet-era fighters on the ground, and the United States detected no radar emissions from any of the air defense sites that it had targeted, military officials said.

The intensity of the attacks startled the 22-member Arab League, which had backed the creation of a no-fly zone. Secretary General Amr Moussa called an emergency meeting of the organization in the wake of a bombardment that he said “led to the deaths and injuries of many Libyan civilians.”

“What is happening in Libya differs from the aim of imposing a no-fly zone, and what we want is the protection of civilians and not the bombardment of more civilians,” said Moussa, according to Egypt’s state news agency.

Mullen said he had seen no evidence that the attacks had caused civilian casualties but charged that Gaddafi had created human shields around radar and missile sites.

The Libyan leader continued voicing defiance and vowed to conduct a “long, drawn-out war.”

“We will not leave our land, and we will liberate it,” he said on state television. “We will remain alive, and you will all die.”

Libya’s armed forces issued a command to all units to observe an immediate cease-fire, a Libyan army spokesman said. An earlier announcement of a cease-fire proved short-lived.

Republican leaders, who had backed military action and have said they would like to see Gaddafi fall, criticized the administration, saying the bombing campaign has no clearly defined goal.

“Before any further military commitments are made, the administration must do a better job of communicating to the American people and to Congress about our mission in Libya and how it will be achieved,” said House Speaker John A. Boehner (Ohio).

Asked on CBS’s “Face the Nation” whether the campaign could lead to a stalemate, Mullen left open the possibility that Gaddafi could remain in power.

“He’s a thug. He’s a cagey guy. He’s a survivor. We know that,” Mullen said. “So it’s difficult to know exactly how it comes out.”

Mullen said on NBC’s “Meet the Press” that the campaign would not overtax the military, despite “the challenges and stress that are presented broadly across the force.” Even though the United States has nearly 100,000 ground troops in Afghanistan and close to 50,000 soldiers in Iraq, the strain on the force in those conflicts has been borne disproportionately by the Army and Marine Corps. The intervention in Libya has fallen largely on the Navy and Air Force.

Democrats said Sunday the intervention was designed to be limited and focused on humanitarian objectives.

“The goal of this mission is not to get rid of Gaddafi, and that is not what the U.N. licensed. And I would not call it going to war,” Sen. John F. Kerry (D-Mass.), chairman of the Senate Foreign Relations Committee, said on “Meet the Press.” “This is a very limited operation that is geared to save lives, and it was specifically targeted on a humanitarian basis.”

Secretary of Defense Robert M. Gates said the United States would cede command of the military operation to allied countries as soon as possible. “We expect in a matter of days to be able to turn over the primary responsibility to others,” said Gates, who spoke to reporters on his military aircraft shortly after he departed Washington for Russia. “We will continue to support the coalition, we will be a member of the coalition, we will have a military role in the coalition, but we will not have the preeminent role.”

The question of who would take over the lead responsibility from the United States, however, remained up in the air. Gates said one possibility would be to allow Britain and France to take leadership, while another would be to run the mission under the aegis of NATO, even though some NATO members, including Germany and Turkey, have expressed reservations.

Gates also cautioned that the military operation should not target Gaddafi personally or exceed the mandate approved by the U.N. Security Council. The council has not called explicitly for Gaddafi’s removal, and only France has given official recognition to the Libyan rebels. “The key is, first of all, to establish the no-fly zone, to do what we can to prevent him from using his military forces to slaughter his own people,” Gates said.

“This is basically going to have to be resolved by the Libyans themselves,” he added.

Finn and Branigin reported from Washington. Correspondent Edward Cody in Cairo and staff writers Liz Sly in Tripoli, Greg Jaffe in Washington, Tara Bahrampour in Marsa Matrouh, Egypt, Keith Richburg in China, and Craig Whitlock, traveling with Gates, contributed to this report.