Iran said it had acted to avenge the US killing of top Iranian general Qasem Soleimani last week, warning it would hit back even harder if Washington responded.

- 'Prepared for anything' -

US President Donald Trump said Iran "appears to be standing down" following the Iranian missile strike.

"All of our soldiers are safe and only minimal damage was sustained at our military bases. Our great American forces are prepared for anything," he said in an address from the White House.

Trump also announced the US would be imposing "additional punishing sanctions" on Iran.

- 'Refrain from further violence' -

NATO chief Jens Stoltenberg said: "I condemn the Iranian missile attacks on US and coalition forces in Iraq. NATO calls on Iran to refrain from further violence."

A NATO official said none of its troops in Iraq had been hurt in the strikes. The alliance had said Tuesday it would take some personnel out of the country for safety reasons.

- 'Resounding blow' -

Prime Minister Benjamin Netanyahu, who has described Soleimani as Iran's "terrorist-in-chief", made it clear Israel would strike back if attacked.

"Anyone who attacks us will receive a resounding blow."

- 'Prioritise diplomacy' -

Russian President Vladimir Putin and Turkish President Recep Tayyip Erdogan issued a joint statement following talks in Istanbul.

"We believe that exchange of attacks and use of force by any party do not contribute to finding solutions to the complex problems in the Middle East," they said.

"We express our commitment to de-escalate the existing tensions in the region and call on all parties to act with restraint as well as commonsense and to prioritise diplomacy."

- 'Urgent de-escalation' -

"Iran should not repeat these reckless and dangerous attacks but should instead pursue urgent de-escalation," Britain's Prime Minister Boris Johnson told parliament.

Foreign Secretary Dominic Raab warned that another war in the Middle East would only benefit the Islamic State group "and other terrorist groups".

- 'No-one's interest' -

EU foreign policy chief Josep Borrell said the attack was yet another example of "escalation and increased confrontation".

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

EU foreign ministers will hold emergency talks on the Iran crisis Friday.

- 'Restraint and responsibility' -

French Foreign Minister Jean-Yves Le Drian said in a statement: "The priority is more than ever for a de-escalation.

"France remains determined to work to ease tensions and is in contact with all the parties to encourage restraint and responsibility."

- 'Logic of dialogue' -

Germany called on Iran to end a "spiral" of conflict and condemned the Iranian attack.

Chancellor Angela Merkel's spokesman Steffen Seibert told reporters all parties needed to "exercise restraint and to turn away from a logic of escalation and towards a logic of dialogue".

Germany temporarily withdrew 32 of its soldiers from a camp close to Baghdad on Tuesday.

- 'Battlefield for warring sides' -

Iraq's President Barham Saleh said he feared "dangerous developments" in the region.

"We denounce the Iranian missile bombing that hit military installations on Iraqi territory and renew our rejection of the repeated violation of state sovereignty and the transformation of Iraq into a battlefield for warring sides," his office said in a statement.