Forces loyal to Libyan leader Moammar Gaddafi went back on the offensive Thursday, as questions continued to mount about the credibility and effectiveness of NATO’s no-fly zone and campaign of airstrikes.

A senior U.S. general described the situation in Libya as a stalemate, while Turkey said it was talking to both sides and working on a “road map” for a cease-fire. In the meantime, Gaddafi is seeking what military advantage he can get and probing for gaps in NATO’s resolve.

At the organization’s headquarters in Brussels, NATO ambassadors held an unscheduled meeting Thursday to follow up on complaints from French Foreign Minister Alain Juppe that the Libya campaign risks getting bogged down unless the pace and efficiency of air support for rebel forces picks up.

The inability of either side to score a decisive victory has left the Obama administration and NATO in a quandary, facing decisions about whether to continue the mission of trying to protect civilians or to increase assistance to the opposition, aid that is currently limited to strikes from air and sea.

Attacks by Gaddafi’s forces began with strikes on desert oil installations that serve as the rebels’ economic lifeline, and they intensified Thursday with the fresh artillery bombardment of rebel positions in the eastern port of Ajdabiya, which sent many fighters fleeing.

The day also ignited new confusion and outrage among rebels in Ajdabiya after warplanes strafed rebel forces and killed at least five people, including two doctors. Rebels first accused NATO of targeting them but later said the attack probably came from Gaddafi’s forces. By Thursday night, it was still unclear who attacked the rebels from the sky.

Abdul Fattah Younis, the rebels’ commander, told reporters that if NATO had attacked their tanks, it was a mistake, and if Gaddafi’s airplanes had been allowed to strike them, it was an “even bigger mistake.”

Either way, NATO’s credibility among rebel forces, already battered since the United States took a back-seat role, appears to have sustained another blow. Rebels are questioning NATO’s resolve to help them.

The government attacks on oil installations in the remote southern desert appeared intended to take advantage of the limits of NATO’s involvement. Even as the rebels made their first oil shipment, a series of attacks on oil installations shut down production at the country’s main oil field of Sarir. An oil company official in rebel-held territory joined the calls Thursday for better protection from NATO.

Rebel fighters in Ajdabiya have grown accustomed to the Western alliance controlling the skies, so they were taken off guard Thursday when low-flying planes fired upon several tanks and a passenger bus loaded with fighters. Younis, the rebel commander, denounced what he called “a vicious attack” and said that the precision of the strikes led him to believe that NATO was responsible.

Outraged rebel fighters called the attack a repeat of an incident last Friday in which NATO bombs mistakenly killed 13 rebels and injured seven others. That incident was triggered when the rebels fired their weapons into the air in celebration — an act that NATO forces mistook for hostile fire.

This time, Younis said, the rebel army had informed NATO of its plan to move tanks and other forces into new positions outside Ajdabiya. The tanks and bus were parked, other fighters said, and were marked with the green, black and red rebel flag.

Rebel forces, meanwhile, came under fire from government loyalists at Ajdabiya’s western gate and rapidly retreated. Many fighters, and some of the few families who had not yet fled the city after weeks of fighting, drove north and east toward Benghazi, the rebel capital, their pickup trucks and cars filled with everything from mattresses to suitcases to automatic weapons.

The main hospital in Ajdabiya was evacuated, with its patients and staff also headed to Benghazi. But Gaddafi’s forces appeared not to have entered the city proper, and some rebel fighters remained.

In Washington, Gen. Carter F. Ham, who commanded the coalition operation until it was taken over by NATO last week, responded affirmatively when asked during congressional testimony Thursday whether the conflict had reached a stalemate. He said that “debate is occurring within the U.S. government” about how best to respond.

In response to a question from Sen. John McCain (R-Ariz.), Ham said he agreed that a stalemate seemed “more likely” than it had been when the United States and its allies began their military strikes last month.

The NATO meeting in Brussels was convened in response to complaints from France, which, along with Britain, has carried out the largest number of sorties over Libya since U.S. forces turned over operational command March 31.

NATO officials said bad weather had reduced visibility and not made it easy to supply the sustained, close air support demanded by rebel commanders. They also accused Gaddafi’s forces of dispersing troops, tanks and artillery among civilian populations in several cities.

The alliance said it was investigating the initial rebel version of what happened near Ajdabiya, but it did not reveal whether coalition warplanes were in the area at the time.

The alliance said that fighting there had been “fierce” for several days and that the battlefield remains confused and disorganized.

“The situation is unclear and fluid, with mechanized weapons traveling in all directions,” said a statement from NATO facilities in Naples.

With a quick military solution looking less likely by the day, Turkish Prime Minister Recep Tayyip Erdogan said his country was holding talks with both sides in Libya and working on a “road map” to achieve a cease-fire.

In any prolonged stalemate, the rebels’ ability to shore up their region’s tattered economy with oil revenue will be critical. Rebels have about 2 million barrels of crude oil in Tobruk that can be exported, but production at the Sarir and Misla fields has halted after a series of attacks.

Two employees of Arabian Gulf Oil Co. are still missing after Gaddafi forces attacked the Misla field with rockets, setting fire to at least one oil tank, a company spokesman, Abdeljalil Mayuf, told the Reuters news agency on Thursday.

Gaddafi’s government has routinely denied attacking oil facilities and has blamed rebels or NATO for the attacks.

“If we get Gaddafi’s forces out of these areas, we can try to reopen Sarir field, but it’s not safe now,” Mayuf said, appealing for air support from NATO.

Denyer reported from Tripoli. Staff writers Karen DeYoung and Walter Pincus in Washington and correspondent Edward Cody in Paris contributed to this report.