A U.S. F-16 fighter fired a missile at a surface-to-air missile installation in southern Iraq Tuesday after Iraqi radar locked on allied patrol planes, Pentagon officials said. Iraq denied any aggressive act.

The fighter had been accompanying four British Tornadoes and other allied warplanes enforcing the southern no-fly zone. All planes returned safely to base.The Iraqi radar at the site had illuminated the British jets, signaling an intention to fire, said Pentagon spokesman Col. Richard Bridges.

A White House official traveling in China with President Clinton said the patrol involved 10 U.S.-allied planes.

In Baghdad, an Iraqi official called the incident proof of the aggressiveness of Americans. "This is an unjustified, aggressive act. No radar was opened," an official at the Ministry of Culture and Information said on customary condition of anonymity.

Vice President Al Gore, speaking to reporters at the White House, said there should be no rush to assume the incident was a deliberate provocation. "We do know from the history of patrolling the no-fly zones that there are a lot of incidents like this from time to time and there are other possible explanations," he said.

"But just so the message is clear, we are going to continue to patrol, and anytime there is any kind of threatening act we will take decisive action," Gore said.

The incident occurred near Basra in southern Iraq at about 1:30 a.m. EDT, during daylight hours in Iraq. The firing of the missile was standard response to Iraq's locking its radar on any U.S.-allied aircraft, the Pentagon said.

There was no report on whether the missile fired by the F-16 struck its target. But British officials said they believe the target was destroyed.

The U.S. aircraft fired because it was armed with the special air-to-ground missiles and had the role of protecting the British aircraft during their patrol mission, Pentagon officials said.

In London, Prime Minister Tony Blair's spokesman said the Iraqi radar locked on to six of the 10 aircraft in the patrol, which included the four British Tornadoes, prompting the U.S. jet to fire.

At the Pentagon, senior military officers, speaking on condition of anonymity, said the incident appeared to have been an isolated one. "We define that (Iraqi) action as `hostile intent,' " a U.S. officer said.

When enemy radar "locks on" or "paints" a warplane, air commanders consider that a precursor to the firing of a surface-to-air missile. During the seven years that U.S. and allied warplanes have patrolled southern Iraq, Iraqi air defenses have periodically been turned on coalition aircraft. Sometimes the action is so brief that no responsive fire is possible.

Tensions with Iraq had subsided in recent months, following a confrontation early this year over Iraqi resistance to U.N. weapons inspections. The United States sent thousands of extra troops to the Gulf before U.N. Secretary-General Kofi Annan defused the situation in a trip to Baghdad in February.