WASHINGTON (AFP) – US fighter jets are still carrying out bombing raids on Libya's air defenses, the Pentagon said Wednesday, days after indicating American combat aircraft had withdrawn from NATO operations.

US warplanes have attacked air defense targets three times since April 4, when NATO assumed command of the air campaign, spokesman Colonel Dave Lapan said in a statement.

US military officials had previously said about 50 combat jets had been pulled back after the handover to NATO, and that air strikes would be carried out by allies while the United States would provide refueling and surveillance aircraft.

It was unclear why the Pentagon had waited to reveal the role of F-16 fighters helping to enforce the no-fly zone, but details emerged amid divisions within the NATO alliance over the air campaign.

Britain and France, which led the calls for international intervention to stop Moamer Kadhafi's attacks on his people, have pressed NATO allies to share more of the burden for the operation and deploy more combat aircraft.

Lapan said the combat flights by F-16 Falcons were confined to striking radar and other air defenses as part of the UN-mandated no-fly zone in Libya.

But US combat aircraft were not part of bombing runs against tanks or other ground targets that fell under a separate UN-approved mission to protect civilians against Kadhafi's forces, he said.

For that mission, American ground-attack aircraft and other warplanes remain on standby pending a request from NATO.

Lapan told reporters earlier that "we have fighter aircraft that NATO has, that they can use as part of the air tasking order for suppression of air defense missions, and they have conducted some of those missions."

The United States had assigned 11 aircraft, including six F-16 fighters and five EA-18 Growlers for electronic jamming, based in Aviano, Italy to target air defenses as part of the no-fly zone mission, he said.

The three strikes by US aircraft were carried out on April 4, 6 and 7, with American pilots flying 97 sorties to suppress the regime's radar and anti-aircraft weaponry since NATO took command, he said.

President Barack Obama's administration, which has nearly 100,000 troops fighting a grinding war in Afghanistan while it tries to wind down the US mission in Iraq, has been eager for Western allies to bear the brunt of the Libya operation.

The Pentagon insisted that the United States was playing a secondary role despite the revelation that American aircraft were still bombing Libyan targets.

"The mission we've been assigned is to provide supporting capabilities to NATO. That's exactly what we are doing with respect to the suppression of enemy air defenses. It is a purely defensive mission," Lapan said in an email.

Defense Secretary Robert Gates and other top officers have said the US military -- which played a dominant role in the opening stage of the Libya operation -- would shift to a focus on mid-air refueling, surveillance flights and search and rescue missions.

Since April 1, the US military has conducted 77 percent of all aerial refueling operations and 27 percent of surveillance and intelligence sorties in the air campaign, according to the Pentagon.

Apart from the F-16 fighter jets and Growler jamming planes, Lapan said the United States had assigned the following aircraft to NATO for Libya operations: 22 KC-135 refueling tankers; a P-3C Orion maritime patrol aircraft; an EP-3E signals reconnaissance aircraft; two E-3 command and control aircraft; two EC-130 signals and communications aircraft; two RC-135 reconnaissance aircraft; a U-2 spy plane, an E-8 surveillance target attack radar system plane; two MQ-1 Predator drones and a Global Hawk surveillance drone.