"They crossed a very big red line by attacking (Iran's) nuclear facilities," Foreign Minister Abbas Araghchi said on the sidelines of a meeting of the Organisation of Islamic Cooperation (OIC) in Istanbul.

He was speaking just hours after President Donald Trump said US warplanes struck three Iranian nuclear sites, nine days into an Israeli bombing campaign targeting its nuclear facilities.

"The most dangerous one happened only last night," Araghchi said, while acknowledging he did not know the full extent of the damage done in the strikes, including one at the underground uranium enrichment facility at Fordo.

"I still do not have exact information about the level of damages, but I don't think it matters... Last night's attack was a grave crime," he said.

"Through this action, the United States has dealt a serious blow to international peace and security," he said, vowing that Iran would defend itself "by all means necessary against... US military aggression".

Araghchi said he would head to Moscow on Sunday and hold talks with Putin on Monday morning.

"I'm going to Moscow this afternoon" to hold "serious consultations with the Russian president tomorrow", he said.

After the strikes, Trump said Iran "must now agree to end this war".

But Araghchi said any demand to return to negotiations was "irrelevant".

"The world must not forget that it was the United States which -- in the midst of a process to forge a diplomatic outcome -- betrayed diplomacy by supporting the genocidal Israeli regime's launch of an illegal war of aggression on the Iranian nation," he said.

"So we were in diplomacy, but we were attacked... They have proved that they are not men of diplomacy, and they only understand the language of threat and force."

Turkey, which was hosting the weekend OIC summit, warned that the strikes risked escalating the Iran-Israel conflict to a global level that could have "catastrophic" consequences.

"The ongoing developments could cause the regional conflict to escalate to a global level. We do not want this catastrophic scenario to come to life," the foreign ministry said in a statement.