WASHINGTON, Jan 5: Four US fighter jets fired air-to-air missiles at Iraqi planes today in a confrontation in the 'no-fly' zone over southern Iraq, a senior US administration official said, reports AP.

The US missiles apparently failed to hit the Iraqi planes, although one Iraqi plane was believed to have crashed, apparently after running out of fuel, the official said.

The American planes - two Air Force F-15 strike aircraft and two Navy F-14 fighters - returned safely after the confrontation, the official said, speaking on condition of anonymity. The Navy planes were from the aircraft carrier USS Carl Vinson.

The incident, which happened at about 2:30 am EST (0730 GMT), marked an escalation of tensions between Iraq and the United States and Britain over enforcing the 'no-fly' zones in northern and southern Iraq. Iraq considers the restrictions illegal and has vowed to defy them.

President Bill Clinton has said US and British forces will continue banning Iraqi flights in the restricted zones, arguing that Saddam Hussein's military still represents a threat to minority Kurds in the north and Shi'ite Muslims in the south.

"The president has made it clear we will enforce the no-fly zones vigorously. The Iraqi violations are at their own peril," David Leavy, spokesman for the White House National Security Council, said today.

A senior Pentagon official, who also confirmed the incident, said the air-to-air altercation was the first in Iraq since December 27, 1992, when a US F-16 shot down an Iraqi MiG-25.

The Pentagon official said he could not identify the number or type of Iraqi aircraft involved in today's incident, but that the Iraqis have MiGs and Mirage F-1s in their current inventory.