WASHINGTON (AP) _ A U.S. F-16 fighter fired a missile at a surface-to-air missile installation in southern Iraq today after Iraqi radar locked on U.S.-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. The 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 in the Roosevelt Room of 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 to respond immediately,'' 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.

No U.S. warplanes were fired upon and there was no report from U.S. officials 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 HARM air-to-ground missiles and had the role of protecting the British aircraft during their patrol mission, Pentagon officials said.

An administration official traveling with President Clinton in Shanghai, China, said the president was informed of the Iraqi incident after he returned to his hotel after a private dinner with Mayor Xu Kuangdi.

By then, at least eight hours had elapsed since the firing. The official did not explain the delay, saying only that Samuel Berger, the White House national security advisor, told the president at his earliest opportunity.

In London, the British Defense Ministry confirmed the incident.

``The Defense Ministry can confirm that a coalition aircraft launched a missile at an Iraqi defense system this morning when that system threatened coalition aircraft on a patrol in the south no-fly zone,'' a ministry statement said.

Prime Minister Tony Blair's spokesman, speaking as is customary on condition of anonymity, 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 have no idea what the motivation might have been,'' said one.

Even so, U.S. and allied aircraft in the region are trained to respond.

``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.

The F-16 fired a high-speed anti-radiation, or HARM, missile.

The incident occurred while coalition aircraft were conducting Operation Southern Watch, the name given to the deny-flight mission, which was established by allies who fought in the 1991 Persian Gulf War.

All the coalition aircraft returned to their bases in Saudi Arabia without further incident. Central Command, a Tampa, Fla.-based headquarters, plans to order an immediate resumption of the patrols.

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, plus warplanes and naval power, to the Gulf before U.N. Secretary-General Kofi Annan defused the situation in a trip to Baghdad in February.

Just last month President Clinton decided to cut the number of U.S. forces in the Gulf from about 37,000 to around 20,000, and on June 4 the Air Force announced it was pulling its F-117 stealth fighter-bombers out of the Gulf area.

There are 20,200 U.S. military personnel, 17 ships and 162 American aircraft in the Persian Gulf region.