WASHINGTON - A key target of U.S. and British bombing in Iraq in recent weeks has been an air base south of Baghdad that would be central to Saddam Hussein's defense against an American invasion. 

Since mid-September, Tallil Air Base - a key link in an Iraqi air defense network that remains formidable despite damage from years of periodic U.S. bombing - has been struck six times, more than any other target in that period, according to Central Command, the headquarters for U.S. operations in the Persian Gulf. 

The choice of bombing targets could reflect Pentagon efforts to lay the groundwork for an invasion if President Bush decides military force is needed to oust Saddam Hussein. Pentagon officials, however, say the attacking pilots are simply responding in self-defense to provocations from Iraqi air defense guns and radars. 

Although Tallil has been a frequent target lately, the bombing has not been extensive enough to neutralize the target. Over the years, Iraq has shown a remarkable ability to repair and replace damaged air defenses. 

Besides Tallil, the other major air defense sites in southern Iraq that have been hit recently are Al Kut, Al Amarah and the airport at Basra. On Tuesday, the Central Command said allied aircraft bombed a command and control communications facility near Al Kut, in response to unspecified "hostile acts" by Iraq. 

Tallil, about 160 miles southeast of the Iraqi capital, is an air defense sector headquarters. It has surface-to-air missiles and the communications facilities to link them to the rest of Iraq's air defense network. It also has two substantial runways and can support dozens of fighters. 

In contrast to the recent flurry of allied attacks in the south - 23 since Aug. 27 - there were none reported in northern Iraq in that period. 

Defense Secretary Donald H. Rumsfeld says the U.S. attacks are simply a necessary response to Iraqi efforts to shoot down the U.S. and British pilots who patrol the skies over northern and southern Iraq to enforce "no fly" zones. Iraq has long asserted that the flight zones are a violation of its sovereignty. 

Rumsfeld did acknowledge last month that he ordered U.S. forces to take a different approach. Instead of firing mostly at Iraqi air-defense guns and radars, pilots are now targeting more of the communications centers, command buildings and fiber-optic links that are easier to find and harder to replace. 

In at least a few cases, U.S. targets have appeared related to preparations for war. 

On Sept. 5, for example, allied pilots bombed a military airfield 240 miles west of Baghdad. The target, as described by Central Command, was ordinary: an air defense command and control facility. But the location was unusual: a remote airfield known as H-3 that originally was built to support an oil pumping station near the Jordanian border. In a break with its usual practice, Central Command did not identify the location. 

Stephen H. Baker, a retired Navy rear admiral who served aboard the USS Theodore Roosevelt during the Gulf War, said the strike at H-3 was unprecedented in the decade-long history of "no fly" zone patrols. 

"The objective of the strike could have been to destroy air defenses to allow easy access for special operations helicopters to fly into Iraq via Jordan or Saudi Arabia as part of a critical primary mission to hunt down Scud" missiles, Baker said recently. "Knocking out Iraqi radars at H-3 also would allow allied aircraft mounting major raids on Iraq a clear route into the country." 

The spate of aerial attacks on the Tallil base began Sept. 15. 

In its typically cryptic description of U.S. and British bombing, the Central Command said precision-guided munitions struck an air defense communications facility at Tallil that day. Nine days later, other unspecified "air defense facilities" were bombed at Tallil and Al Amarah, another repeated target. 

Tallil was targeted by allied bombers during the 1991 Gulf War, in part because it was considered a probable storage site for chemical weapons. It reportedly served as a staging point for Iraq's airborne chemical attacks against Iran in the 1980s. Post-Gulf War U.N. inspections found no evidence of chemical weapons there. 

Al Amarah has an air defense base and headquarters for the Iraqi Army's 4th Corps. It is on the Tigris River, about 165 miles southeast of Baghdad. 

On Sept. 27, allied planes again attacked Tallil, this time targeting what Central Command described as a surface-to-air missile control radar and a surface-to-air missile launcher. On the same mission, Al Amarah was hit again; Central Command reported targeting an air defense operations center there but gave no details. 

The next attack on Tallil was Oct. 3. Central Command said the targets were an air defense sector headquarters building and an integrated operations center. Surface-to-air missile sites at Tallil were bombed on Oct. 10 and 11.