An AC-130 gunship is shown in this undated photo provided by the U.S. Air Force. (ASSOCIATED PRESS)

The U.S. military dramatically stepped up its assault on Libyan government ground forces over the weekend, launching its first missions with AC-130 flying gunships and A-10 attack aircraft designed to strike enemy ground troops and supply convoys.

The use of the aircraft, during days of heavy fighting in which the momentum seemed to swing in favor of the rebels, demonstrated how allied military forces have been drawn deeper into the chaotic fight in Libya. A mission that initially seemed to revolve around establishing a no-fly zone has become focused on halting advances by government ground forces in and around key coastal cities.

The AC-130s, which fly low and slow over the battlefield and are typically more vulnerable to enemy fire than fast-moving fighter jets, were deployed only after a week of sustained coalition attacks on Libyan government air defenses and radar sites. These aircraft, armed with heavy machine guns and cannons that rake the ground, allow strikes on dug-in Libyan ground forces and convoys in closer proximity to civilians.

The planes are being used to step up pressure on Libyan ground troops, who have retreated from the rebel’s advance and fortified around several cities east of Tripoli, the capital. “Our strategy continues to be to pressure them where we think it’s going to give us the best effect,” said Vice Adm. William Gortney, director of the Pentagon’s Joint Staff, referring to the use of the new aircraft. “The number of the strike sorties that you saw, I think, is a direct result of that.”

Gortney emphasized that the military was not using the planes to facilitate a rebel advance. The Washington Post learned of their deployment last week but withheld reporting the information until their first missions at the request of U.S. military officials.

Military officials consider AC-130s and A-10s well suited to attacks in built-up areas, although their use has led to civilian deaths. Unlike fighter jets and bombers, which typically carry 500- or 1,000-pound bombs, the AC-130s and A-10s deliver more discriminate but still devastating machine-gun fire. “They offer weapons that you can meter against a much smaller area and not risk as much collateral damage,” said retired Air Force Lt. Gen. David A. Deptula, who played a key role in overseeing the initial U.S. attack on Afghanistan in 2002.

AC-130s were used to great effect during the two U.S. offensives in Fallujah, a stronghold of the insurgent group al-Qaeda in Iraq in the early days of the Iraq war. In Afghanistan, the military considers them particularly effective against entrenched militants, and commanders have frequently complained that they are in too short supply. The gunships, developed from a Hercules C-130 transport plane for use in Vietnam, put pilots at greater risk than fighter jets, but they have been used in virtually every U.S. military combat operation since then.

In Libya, “we are determined to step up the mission, to attack his tanks and [troop] columns every day until he withdraws,” a French official said of Libyan leader Moammar Gaddafi and the forces loyal to him.

The AC-130s, which are flying from a base in Italy, were requested by Gen. Carter Ham, the senior U.S. general overseeing the operation, and are likely to continue flying over Libya in the coming days as allied forces attempt to increase the pressure on Gaddafi’s ground forces.

“The longer it lasts, the more danger of civilian casualties,” said a Western diplomat whose country is involved in the attacks. He warned that one errant missile strike against a hospital or a house full of children could have a deeply polarizing effect on the fragile alliance of NATO and Arab nations.

The tougher and more risky mission to stop Gaddafi’s ground troops from attacking key cities has quickly overshadowed the less challenging task of stopping the Libyan dictator from launching his aircraft to attack rebels. The ground attack mission also opened up some rifts among coalition partners in NATO and Arab nations, which were reluctant to support attacks that could cause civilian casualties. And it has led some U.S. lawmakers to accuse the Obama administration of inserting the U.S. military in the middle of a complex ground fight between rebels and loyalist forces without a clear exit strategy.

On Monday, Russian Foreign Minister Sergei Lavrov said strikes on Gaddafi’s forces would amount to taking sides in what he called a civil war. He said such attacks would breach a U.N. mandate authorizing intervention in Libya that was initially envisaged as establishing a no-fly zone only to protect civilians.

In discussions that began in late February, NATO planners focused primarily on providing humanitarian support, enforcing an arms embargo and establishing the no-fly zone to prevent Gaddafi from using his aircraft to attack his people, according to a senior NATO diplomat.

Separately, the United States, Britain and France made preparations for stopping a ground assault by Libyan forces. There was little support within President Obama’s national security team for a mission that revolved solely around a no-fly zone seen as likely to do too little to protect civilians against Gaddafi’s forces.

Some administration officials, with memories of enforcing no-fly zones over Bosnia while civilians were being exterminated on the ground, said the United States should not participate in such a limited operation. At the , there was concern about plunging U.S. forces into a conflict without a clear goal, and there was worry that chaos would ensue if Gaddafi fell too quickly and the loosely organized, tribally divided rebels tried to govern the country.

But by March 12, the Arab League had formally backed the imposition of a no-fly zone, after a similar move by the Gulf Cooperation Council, which consists of several of the United States’ closest Arab allies.

Until the week of March 13, the rebels seemed to be making progress. Then Gaddafi unleashed his military, taking towns that the opposition had won and heading toward the de facto rebel capital, Benghazi.

Pushed by Secretary of State Hillary Rodham Clinton and U.N. Ambassador Susan E. Rice, the administration took control of a British-French draft resolution for a no-fly zone and began making the case to the rest of the Security Council that stronger action was needed. The resolution passed March 17, authorizing the use of “all necessary measures” to protect civilians and civilian areas under threat.

“In an ideal world, we would sit down with a blank sheet of paper and say, ‘Let’s get rid of Gaddafi.’ That’s not the way it unfolded,” said the Western diplomat whose country is involved in the Libya mission.