Tehran fired ballistic missiles at two bases in Iraq housing coalition forces in the first act of the Islamic republic's promised revenge for the killing of one of its top generals in a US drone strike.

The assassination of Qasem Soleimani in Baghdad on Friday sparked an outpouring of rage and grief in Iran, and the country's supreme leader hailed the latest rocket attacks as a "slap in the face" for Washington.

"The latest rocket attacks on air bases in Iraq used by US and coalition forces, among them European forces, are yet another example of escalation and increased confrontation," EU foreign policy chief Josep Borrell told reporters.

"It is in no-one's interest to turn up the spiral of violence even further," he said, warning that the latest crisis was hampering the fight against the Islamic State.

EU foreign ministers are holding emergency talks on the Iran crisis on Friday to discuss what the bloc can do to reduce tensions, but its options for action are limited.

European Commission President Ursula von der Leyen, who will raise the situation when she meets British Prime Minister Boris Johnson in London later Wednesday, said "the use of weapons must stop" to allow space for dialogue.