WASHINGTON - US President Donald Trump said Saturday that Iran will face more military strikes if peace does not come quickly.

Addressing the nation Saturday evening, Trump said that Iran's key nuclear facilities had been "completely and totally obliterated," and that future attacks will be "far greater and a lot easier."

"If peace does not come quickly, we will go after those other targets with precision, speed and skill. Most of them can be taken out in a matter of minutes," he said.

Trump said that he decided a long time ago not to let Iran gain a nuclear weapon and that without peace, there will be "tragedy" for Iran that will far exceed what's taken place.

"War starts right now," Iran's Islamic Revolutionary Guard Corps said after Trump's announcement of US successful attacks on three Iranian nuclear facilities of "Fordow, Natanz and Esfahan."

Iran had evacuated these three nuclear sites "a while ago," said Hassan Abedini, deputy political director of Iran's state broadcaster.

Appearing on state-run television, he said that Iran "didn't suffer a major blow because the materials had already been taken out."

Iran's Atomic Energy Organization confirmed the US attacks, but said that its work will not be stopped.

"The Atomic Energy Organization of Iran assures the great Iranian nation that despite the evil conspiracies of its enemies, with the efforts of thousands of its revolutionary and motivated scientists and experts, it will not allow the development of this national industry, which is the result of the blood of nuclear martyrs, to be stopped," the agency said in a statement issued after Trump's announcement of the US attacks.

Meanwhile, UN Secretary-General Antonio Guterres said Saturday that US strikes on Iran's nuclear sites are "a dangerous escalation" and "a direct threat to international peace and security."

"I am gravely alarmed by the use of force by the United States against Iran today. This is a dangerous escalation in a region already on the edge -- and a direct threat to international peace and security," said Guterres in a statement.

There is a growing risk that this conflict could rapidly get out of control -- with catastrophic consequences for civilians, the region and the world, he said.

Guterres called on UN member states to uphold their obligations under the UN Charter and other rules of international law.

"At this perilous hour, it is critical to avoid a spiral of chaos. There is no military solution. The only path forward is diplomacy. The only hope is peace," he said.