US President Donald Trump said Tuesday that a ceasefire between Iran and Israel was holding, shortly after he lashed out at both countries and cursed as he accused them of violating the truce.

In a fast-moving series of declarations, the 79-year-old Republican, who was on his way to attend a NATO summit in The Hague, posted on his Truth Social app that "the Ceasefire is in effect!"

"ISRAEL is not going to attack Iran. All planes will turn around and head home, while doing a friendly 'Plane Wave' to Iran. Nobody will be hurt," Trump wrote.

Minutes earlier, he had castigated Iran and also close US ally Israel for violating a ceasefire he had originally announced late Monday.

The two countries have been "fighting so long and so hard that they don't know what the fuck they're doing, do you understand that?" the president told reporters at the White House.

Iran violated the ceasefire, "but Israel violated it too," Trump told reporters on the White House's South Lawn as he departed for the NATO summit.

"So I'm not happy with them. I'm not happy with Iran either. But I'm really unhappy if Israel is going out this morning."

"I've got to get Israel to calm down," he said. "Israel, as soon as we made the deal, they came out and dropped a load of bombs the likes of which I've never seen before."

Trump's unusually public display of anger at Israel saw the US leader apparently trying to cajole his ally to call off warplanes in real time.

Earlier the same morning, he had posted on Truth Social: "ISRAEL. DO NOT DROP THOSE BOMBS" -- without it being clear which bombs he was referring to.

"IF YOU DO IT IS A MAJOR VIOLATION. BRING YOUR PILOTS HOME, NOW!"

If it holds, the truce would be a big political win for Trump in the wake of his risky decision to send US bombers over the weekend to attack three nuclear facilities in Iran that Israel and the United States say were being used to build an atomic bomb in secret.

The US leader had said the truce would be a phased 24-hour process beginning at around 0400 GMT Tuesday, with Iran unilaterally halting all operations first. He said Israel would follow suit 12 hours later.

Israel has been bombing Iran in an offensive that began June 13. The United States joined the attack with a mission starting overnight Friday to Saturday against the deeply buried Fordow complex and two other sites.

Speaking to reporters at the White House, Trump insisted that the US bombing mission was a success.

"I think it's been completely demolished," he said, savaging US journalists for "fake news" and calling two networks "scum" for reporting that it remains unclear whether the Iranian nuclear infrastructure was truly dismantled.

"IRAN WILL NEVER REBUILD THEIR NUCLEAR FACILITIES!" he posted separately on Truth Social.