Defense Secretary Donald H. Rumsfeld said Monday he cannot imagine a formula for U.N. weapons inspections that would be both acceptable to Iraq and successful in uncovering nuclear weapons.

He did not say what should be done in the absence of effective inspections, but Rumsfeld in the past has endorsed the view that if the goal is to stop Iraqi President Saddam Hussein from threatening to use a weapon of mass destruction then military action would be more effective than diplomacy.

"Everyone knows" Iraqi President Saddam Hussein is pressing ahead with a nuclear program and striving to improve and expand his chemical and biological weapons arsenal, Rumsfeld told a Pentagon news conference.

In a related development, two U.S. Air Force F-16s dropped laser-guided bombs on an air defense radar complex in southern Iraq on Monday. Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said the F-16s attacked after one of the Iraqi radars "illuminated" one of the planes.

It was the first U.S. airstrike in southern Iraq since Jan. 21.

In Baghdad, the state-run Iraqi News Agency quoted an unidentified army spokesman as saying "enemy warplanes bombed civil and service installations in southern Iraq." The report gave no other details.

To lift the veil of secrecy from Saddam's work on weapons of mass destruction would require an inspection system that is "enormous intrusive" -- more so than anything tried in the past, he said.

"I just can't quite picture how intrusive something would have to be that it could offset the ease with which they have previously been able to deny and deceive, and which today one would think they would be vastly more skillful, having had all this time without inspectors there," Rumsfeld said.

Amid growing speculation that the United States may launch a military offensive in Iraq to oust Saddam, the Iraqi government is negotiating with U.N. Secretary General Kofi Annan on a resumption of weapons inspections, which ended in 1998 after Saddam refused to cooperate with the inspectors' demands.

U.N.-Iraq talks scheduled for next week have been postponed at Iraq's request.

Rumsfeld said U.N. inspections in Iraq that began after the 1991 Gulf War were inadequate.

"They did a lot of looking around, and they found some things," he said. "But for the most part, anything they found was a result of having been cued to something as a result of a defector giving them a heads-up that they ought to do this, that or the other thing."

In the years since inspectors left Iraq in 1998, the Iraqis have only increased their ability to hide their work, he said. He cited an "enormous amount" of equipment that Iraq has acquired "enabling them to become more mobile, enabling them to go underground to a greater extent than they had been previously."

In January 1998, Rumsfeld and Paul Wolfowitz, who is now deputy secretary of defense, signed an open letter to then-President Clinton that said it was nearly impossible to adequately monitor Saddam's weapons programs. It said the only acceptable strategy is to "eliminate the possibility" that Iraq could use or threaten to use a weapon of mass destruction, and that this would require military action.

In a related matter, Rumsfeld denied a Washington Post report that Wolfowitz had asked the CIA to investigate the performance of Swedish diplomat Hans Blix, chairman of a U.N. team formed to resume weapons inspections in Iraq. Blix previously headed the International Atomic Energy Agency.

Rumsfeld would not say whether Wolfowitz had asked the CIA for information on Blix but denied there was any request for an investigation.