WASHINGTON — U.S. warplanes have bombed three ground targets in Libya since the Obama administration announced early this month that the United States was shifting to a support role in the NATO-led air campaign, the Pentagon acknowledged Wednesday.

Col. Dave Lapan, a Pentagon spokesman, said that U.S. fighter jets have attacked Libyan air defense sites as part of the no-fly zone imposed last month under a U.N. mandate to deter attacks against civilians by forces loyal to Libyan leader Moammar Gadhafi.

Administration officials have said previously that the U.S. had halted strike sorties on April 4, several days after turning over command of the air campaign to the NATO alliance, and that attacks on Libyan tanks and other ground units would be handled by aircraft from Britain, France and other allies.

But the limited U.S. role has grown increasingly controversial in recent days.

Libyan opposition leaders and some NATO allies publicly complained about the intensity and scope of the air campaign against Gadhafi’s forces since the U.S. announced that American warplanes would cease participating in the mission to protect civilians.

In his speech last month announcing the turnover to a non-U.S. NATO commander, President Barack Obama said, “”The United States will play a supporting role — including intelligence, logistical support, search and rescue assistance, and capabilities to jam regime communications.””

A senior military official said that three attacks were launched last week against Libyan surface-to-air missile sites. U.S. F-16 and EA-18G fighters carried out the strikes with 500-pound bombs.

Lapan disputed suggestions that the attacks contradicted administration claims to have assumed a support role in the operations. He said the U.S. combat role was confined to enforcing the no-fly zone and that non-American warplanes were attacking Libyan military units that threatened civilians.

“”Having a few aircraft in this capacity (conducting airstrikes) does not change”” the fact that the U.S. is in a support role, Lapan said.

But the Pentagon appeared eager Wednesday to counter the impression that the U.S. had withdrawn completely from an active role in the air campaign.

Of the 134 missions to monitor against Libyan air defense sites since NATO took command, 97 of them have been flown by U.S. aircraft.

Lapan added that, since April 1, U.S. aircraft have carried out 35 percent of the sorties, 77 percent of the aerial refueling missions and 27 percent of the surveillance flights.

U.S. officials have said that other fighters and ground attack planes remain available to NATO, but that Defense Secretary Robert M. Gates would have to approve any request to use them to attack ground forces. No such requests have been received, Lapan said.