The United States and Israel crossed a major red line in attacking Iran's nuclear facilities, Iran's top diplomat warned on Sunday, saying he was heading to Russia for talks with President Vladimir Putin.

"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, vowing that Iran would defend itself "by all means necessary against, not just US military aggression, but also the reckless and unlawful actions of the Israeli regime."

Uncertainty over nuclear talks

He said he would head to Moscow on Sunday and hold talks with Putin on Monday morning in the wake of the unprecedented strikes.

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

After the strikes, Trump said that Iran "must now agree to end this war" and that under no circumstances could Iran possess a nuclear weapon.

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

US 'betrayed diplomacy': Iran

"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 gave a green light to Israelis, if not instructed them, to attack Iran's nuclear facilities. They have proved that they are not men of diplomacy, and they only understand the language of threat and force."