U.S. pilots patrolling the so-called no fly zones in northern and southern Iraq are taking a new approach to target selection, to defend themselves against Iraqi gunners.

Defense Secretary Donald Rumsfeld says he ordered the change in tactics more than a month ago, in an effort to do more lasting damage to Iraq's air defense system, and thus provide greater protection to U.S. pilots being targeted by Iraqi gunners.

General Peter Pace, Vice Chairman of the Joint Chiefs of Staff of the U.S. Armed Forces, says the change has involved the targeting of hardened command-and-control facilities and airfields, versus the individual mobile guns and radars tracking and firing on American flyers.

General Pace says U.S. counterstrikes have succeeded in degrading Iraq's air defenses.

But Defense Secretary Rumsfeld cautions, Iraq has a history of rebuilding and improving its air defenses, making it difficult to say if Baghdad is now less capable of defending itself against any possible U.S. military invasion.

"Whether they are going to be net stronger or weaker in the event anything were to occur in the future is a function of how fast they are able to rebuild, replace and replenish that capability," Secretary Rumsfeld said.

U.S. and British aircraft patrolling the southern no-fly zone have appeared particularly active in recent weeks, hitting targets in the south five times already this month, following eight strikes in August. No coalition planes have been downed or damaged by Iraqi fire.

Nevertheless, Pentagon officials insist there has been no increase in air activity overall this year.

Mr. Rumsfeld says he will appear before Congress later this week, in a bid to make a case for pre-emptive action against Baghdad.

Mr. Rumsfeld says that to wait for a so-called smoking gun, or clear-cut evidence of the threat posed by Iraq and its weapons of mass destruction would be a mistake.

"There isn't a single smoking gun that everyone nods and says 'Ah ha, that's it.' If we wait for a smoking gun in this instance, it obviously would be after the fact," he said. "You'd find it after the fact. You'd find it after lethal weapons were used against the United States, our friends and allies."

In the meantime, Mr. Rumsfeld indicates he is heartened by the remarks of a leading Saudi official suggesting that U.S. forces could use bases in Saudi Arabia for any action against Iraq sanctioned by the United Nations. He says it is another indication of the mounting pressure on Iraqi leader Saddam Hussein.