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 seven 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.