American fighter aircraft bombarded a missile battery in northern Iraq yesterday in the most serious clash since the end of Operation Desert Fox eight days ago.
Iraq said later its air defences had probably shot down a western plane and a search was under way to locate the wreckage of the aircraft and its pilot.
The White House dismissed the report as "more Iraqi propaganda". "It's totally not true," said the spokesman, Mr David Leavy, after an Iraqi army communique said it was "virtually certain" that anti-aircraft batteries had shot down an "enemy airplane".
Washington said F-16 jets retaliated after coming under fire from Iraqi ground-to-air missiles. All later landed safely back at their base at Incirlik in Turkey, US officials added.
Iraq, which warned the US and Britain on Sunday that it would fire at planes flying over its territory, said four Iraqis were killed.
A US defence department spokesman said: "We can confirm today there was an aggressive Iraqi action against US fighter aircraft in support of Operation Northern Watch. Our aircraft responded within normal rules of engagement."
"The air crews acted in self-defence and responded with HARM missiles and precision-guided munitions. There was no damage to coalition aircraft and all our crews returned safely to base [in Turkey]," the Defence Department spokesman said.
President Clinton said he was "proud" of US pilots involved in the latest action. They had "attacked because they were attacked", he said. Mr Clinton added that the US would continue to enforce no-fly zones in the north and south of Iraq.
"Because we effectively control the skies over Iraq, [President Saddam Hussein] has been unable to use air power against his own people or lash out again at his neighbours," Mr Clinton said.
An Iraqi military communique said: "Many hostile formations violated Iraqi airspace coming from Turkey . . . They approached our air defences which bravely and capably intercepted and forced [the planes] to flee, returning to the bases of evil and aggression in Turkey."
It said aircraft returned later and "fired their criminal missiles towards one of our positions . . Our air defences forced them to flee . . . These aggressive acts led to the martyrdom of four of our brave fighters and injuries [to another] seven."
Iraq's UN envoy scoffed at US claims that its pilots acted in self-defence. "They said they acted in their own defence - as if Florida was attacked," the ambassador, Mr Nizar Hamdoon, said.
"Iraq has always said that there is no way that it could recognise the no-fly zones. They are illegal and they have not been established by United Nations resolutions," he said.
"Why should there be such a breach of Iraq's sovereignty? Why should Iraq stand idle?"
Asked why the Iraqi guns fired on the planes now when they had been combing the no-fly zones regularly since 1991, Mr Hamdoon replied: "I have no idea about the timing." The zones have not been authorised by the UN Security Council.
Russia, a permanent member of the Security Council, urged restraint after the clash.
A communique stressed the need to "abstain from actions which do not contribute to re-establishing an atmosphere favourable to a political resolution of the Iraqi problem". The communique said the incident "cannot help but provoke preoccupation".
"We feel that after the recent events in the Iraqi crisis, it is more important than ever to not let the situation worsen in this region," it went on.
But the statement's tone was moderate compared to those at the start of the four-day "Operation Desert Fox", which prompted Moscow to recall its ambassadors to London and Washington.