Updated

U.S. warplanes bombed three communications facilities at air defense sites in southern Iraq Wednesday after Iraqi gunners fired missiles and artillery at the American aircraft, officials said.

"It looks like it's a spike" in the long-running pattern of Iraqi challenges to the no-fly zones over northern and southern Iraq, said Rear Adm. David Gove, deputy director of global operations for the Joint Chiefs of Staff.

Pentagon officials said Iraq has stepped up its efforts to shoot down American and British pilots patrolling the "no-fly" zone over southern Iraq as U.N. weapons inspectors prepare to begin their work. U.S. planes have attacked six air defense facilities this week.

Gove said the United States and Britain are sticking to their usual measured response.

"We're responding as we have for previous no-fly zone violations over the last couple of years," Gove said.

In a decade of flying over southern and northern Iraq, no U.S. or British pilot has been shot down. Iraqi gunners rarely use their targeting radars because they are vulnerable to U.S. high-speed anti-radiation missiles.

In Wednesday's confrontation, Iraqi air defenses fired surface-to-air missiles and anti-aircraft artillery at U.S. planes. In response, 12 U.S. planes dropped a total of 20 bombs on three air defense communications relay stations near Al Kut and Basra, a senior U.S. official said, speaking on condition of anonymity.

On Monday, U.S. aircraft, responding to bursts of Iraqi missile and artillery fire, attacked an air defense communications facility and a radar facility near Al Kut and an air defense communications facility at Tallil.

The Bush administration, while seeking to strengthen allied support for a possible invasion of Iraq, is publicly pressing its case that Iraqi President Saddam Hussein is flouting the United Nations by firing on U.S. and British aircraft, pursuing a banned program to develop nuclear weapons and violating a U.N. economic embargo.

In his remarks at the Pentagon, Gove said Iraq is continuing to sell oil in violation of U.N. limits imposed after it invaded Kuwait in August 1990. In the past week, allied vessels enforcing the U.N. economic embargo boarded 118 ships in the northern Persian Gulf and diverted 42 of them for carrying contraband material, Gove said.

Defense Secretary Donald H. Rumsfeld has frequently called Iraq's firing on U.S. and British planes intolerable.

On Tuesday he told reporters that retaliatory strikes on Iraqi anti-aircraft sites would continue.

"You can be absolutely certain we'll not allow our aircraft to continue to be shot at with impunity. We intend to respond," Rumsfeld said.

In preparation for possible war in Iraq, the administration has quietly contacted 50 countries, including Canada and Britain, to solicit contributions of troops and war materiel in the event Bush decides to use force, an administration official said Wednesday on condition of anonymity.

Victoria Clarke, spokeswoman for Rumsfeld, said she could not discuss specifics of the consultations. She said there have been "lots of conversations" — including unsolicited offers to Rumsfeld of assistance from some unidentified countries — about "who can do what" if it comes to war against Iraq.

At the State Department, spokesman Phil Reeker said it was premature to focus on war preparations.

"We're having discussions with many countries," he said. "I don't have a specific list for you."

The Associated Press contributed to this report.