WASHINGTON -- U.S. fighter jets bombed Iraqi air defenses in separate attacks Tuesday in the southern and northern "no-fly" zones, defense officials said.

The official Iraqi news agency said the strike in the south injured four people. The Pentagon generally does not comment immediately on casualties related to patrols over Iraq.

Douglas Feith, undersecretary of defense for policy, told reporters that the latest attacks were a continuation of the administration's policy of responding to provocations from Iraqi air defenses.

"We're not going to just absorb passively these challenges to crews who are enforcing a U.N. policy. They're performing an important function, and the Iraqi government has no right to be shooting at them," Feith said.

He would not say whether the Bush administration was taking a new, more aggressive approach toward Iraq and its president, Saddam Hussein.

"We note that the Saddam Hussein regime is continuing to pursue weapons of mass destruction and missile capabilities. It continues to pose a threat to neighboring states. It is engaged in subversive activities throughout the region; it's working to aggravate Arab-Israeli problems, for example. And it's seeking to undo the no-fly zone policy by continually attacking the coalition aircraft.

"The Saddam Hussein regime is a serious threat to its neighbors and the well-being of its own people, and we will be taking all of this into account. That's all I want to say at the moment," he said.

Four attacks in two weeks

Tuesday's was the fourth attack in southern Iraq in less than two weeks. In a brief announcement, U.S. Central Command said the strike was in response to recent Iraqi "hostile threats" against the American and British aircraft that regularly patrol the skies over southern Iraq.

The announcement gave few details beyond saying the targets were Iraqi anti-aircraft artillery and surface-to-air missile sites.

A defense official, speaking on condition of anonymity, said the targets were near the city of As Samawah, about 130 miles southeast of Baghdad. They were attacked by U.S. Air Force F-16s and U.S. Navy F/A-18s. The Navy jets were launched from the USS Enterprise aircraft carrier in the Persian Gulf.

The attack happened at about 8 a.m.

Radar sites targeted

In the northern zone, U.S. officials said Air Force jets fired high-speed anti-radiation, or HARM, missiles at three Iraqi air defense radar sites north of the city of Mosul. The attack was provoked by Iraqi anti-aircraft artillery fire directed at U.S. aircraft near Mosul, U.S. officials said.

U.S. forces have been attacking Iraqi air defense targets with increased regularity in recent days, particularly in the south.

Last Thursday, four Air Force F-16 fighter jets attacked a long-range radar stationed at Basra airport in southern Iraq. U.S. officials said the radar was not active at the time of the attack but had been used in the past to coordinate Iraqi air defense targeting of U.S. and British aircraft in the area.