Officials in Washington said four US aircraft fired six air-to-air missiles at four Iraqi planes although none hit their targets.

BBC's Philippa Thomas: US may be preparing for further military action

One Iraqi fighter was reported to have crashed. Correspondents said it may have run out of fuel. All US aircraft returned safely to base.

A Pentagon spokesman said there had been a total of eight violations of the southern no-fly zone on Tuesday, involving as many as 14 Iraqi jets. He could not confirm the reports of the crash of the Iraqi plane.

The Iraqi military denied it had lost a plane in the incident. A spokesman told the official Iraqi news agency that Iraqi jets had "confronted" allied aircraft that had "violated" Iraq's air space. The spokesman said all planes returned safely.

"It does appear that Saddam is frustrated because he has not been able to win support from the neighbouring Arab countries and the UN Security Council," Pentagon spokesman Ken Bacon said.

"This type of challenge that we now see in the no-fly zone could be a sign of that frustration," he said.

It was the first clash between Iraqi and US fighters since December 1992, when a US Air Force F-16 Falcon shot down an Iraqi MiG-25 after being illuminated by radar.

The BBC's Caroline Wyatt in Baghdad: No reaction from Iraq

Tuesday's encounter, involving land-based air force F-15s, and Navy F-14 jets from aircraft carrier USS Carl Vinson, happened around 1015 Gulf time (0715 GMT), a Pentagon official said.

A US military official, Major Joe LaMarca, said US warplanes patrolling the zone responded after being "engaged" by a number of Iraqi jets.

Speaking from Central Command headquarters in Tampa, Florida, he said it was the "most aggressive" incident involving Iraqi jets breaching the no-fly zone.

Latest incident

Two no fly zones - one in the north and one in the south of Iraq - were set up after the 1991 Gulf War. They are patrolled by US and UK planes.

Last week, US planes fired on ground missile batteries in both zones after they said missiles had been fired at them from the ground.

The latest incident was the third in less than a week involving US warplanes enforcing the no-fly zone. BBC Washington Correspondent Philippa Thomas said it was the most serious since the US and UK bombed Iraq in December in Operation Desert Fox.

Saddam Hussein: "People of the Arab World, rise against the foreigners"

Following the clash, a speech was broadcast by Iraqi President Saddam Hussein calling for Arabs to overthrow any of their leaders supporting the US. The address, to mark Army Day, was brought forward by a day.

On Monday, White House's National Security Council Spokesman David Leavy said: "The president has made it clear we will enforce the no-fly zones vigorously. The Iraqi violations are at their own peril."

Iraqi air force chief General Khaldoun Khattab Omar said: "Iraqi fighter planes are totally free to fly the skies of Iraq, in the north and the south.

"They have the right to defend our territory and no-one can stop them."