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WASHINGTON (CNN) -- A U.S. F-16 aircraft fired a missile at an Iraqi ground-radar installation early Tuesday morning, launching a change in strategy that gives U.S. pilots "broader latitude" in attacking targets in Iraq's no-fly zones, U.S. officials said.

In Tuesday's incident, a U.S. pilot fired a missile at what was described as an Iraqi early-warning radar site near the city of Mosul in northern Iraq, the second such incident in the northern no-fly zone within 48 hours.

National Security Council and Pentagon officials told CNN that, while the system was not a "targeting radar," it was nevertheless considered "hostile."

The radar was operating at the time of the attack, which occurred about 11 a.m. The pilot returned safely to base in Incirlik, Turkey, from where many of the patrol flights are launched.

An F-16 fired a missile at a ground-radar station in the northern no-fly zone Tuesday

Pentagon officials said the attack represented a new, more aggressive enforcement of the no-fly zones, where pilots are authorized to destroy targets that pose a general threat to U.S. and British planes, instead of being limited to responding only to direct threats or attacks.

The officials insisted the new strategy was not an escalation, as it is permitted under the existing rules of engagement for pilots in the no-fly zones.

Northern Iraq has been patrolled by U.S. and British pilots since April 1991 to protect Kurdish rebels; a southern no-fly zone has been in operation since August 1992, to protect Shiite Muslims.

There have been a number of incidents when pilots were "painted" by enemy radar or were the targets of anti-aircraft fire in the last two weeks.

U.S. Defense Secretary William Cohen, speaking during a visit to Japan, said Iraqi President Saddam Hussein was becoming more "frantic and agitated."

Cohen said the United States would give no ground in enforcing the no-fly zones, and he warned Baghdad it would pay a price if it challenged U.S. and British air patrols.

Cohen declined to say whether repeated Iraqi missile threats against U.S. and British jets, or violations by Iraqi warplanes of the no-fly zones, might prompt wider attacks against the Iraqi military by U.S. forces.

According to Cohen, Hussein is feeling 'frantic and agitated' since the December strikes

"We are going to protect our forces and take whatever measures are necessary," he said. "Beyond that I wouldn't want to say anything."

Cohen said Hussein appeared to feel increasingly cornered since the December 16-19 cruise missile and bombing raids by American and British forces, aimed at punishing Baghdad for failing to cooperate with U.N. weapons inspectors.

"He (Hussein) is lashing out verbally, rhetorically, against the Saudis, against the Egyptians, against the Kuwaitis," Cohen said.

That "and the nonrecognition of the no-fly zones would seem to indicate that he is certainly more agitated and frantic," Cohen said.

Correspondent Jamie McIntyre, The Associated Press and Reuters contributed to this report.