"THE CEASEFIRE IS NOW IN EFFECT. PLEASE DO NOT VIOLATE IT!" Trump said in a Truth Social post on Tuesday.
When Trump announced on Monday what he called a complete ceasefire to end a 12-day war, he appeared to suggest Israel and Iran would have time to complete missions that were under way, at which point the ceasefire would begin in a staged process.
Witnesses said they heard explosions near Tel Aviv and Beersheba in southern Israel before Trump's statement.
Israel's military said six waves of missiles were launched by Iran and Israel's national ambulance service said four people were killed in Beersheba, the first reported deaths in Israel since Trump announced the ceasefire.
Iran's semi-official SNN news agency reported on Tuesday that Tehran fired its last round of missiles before the ceasefire came into effect.
A senior White House official said Trump had brokered a ceasefire deal in a call with Israeli Prime Minister Benjamin Netanyahu and Israel had agreed so long as Iran did not launch further attacks.
An Iranian official earlier confirmed that Tehran had agreed to a ceasefire, but the country's foreign minister said there would be no cessation of hostilities unless Israel stopped its attacks.
Abbas Araqchi said early on Tuesday that if Israel stopped its "illegal aggression" against the Iranian people no later than 4am Tehran time (10.30am AEST) on Tuesday, Iran had no intention of continuing its response afterwards.
Israel, joined by the United States on the weekend, has carried out attacks on Iran's nuclear facilities, after alleging Tehran was getting close to obtaining a nuclear weapon.
Iran denies ever having a nuclear weapons program but Supreme Leader Ali Khamenei has said that if it wanted to, world leaders "wouldn't be able to stop us".
Qatari Prime Minister Sheikh Mohammed bin Abdulrahman Al Thani secured Tehran's agreement during a call with Iranian officials, an official briefed on the negotiations told Reuters on Tuesday.
US Vice-President JD Vance, Secretary of State Marco Rubio and US special envoy Steve Witkoff were in direct and indirect contact with the Iranians, a White House official said.
Hours earlier, three Israeli officials had signalled Israel was looking to wrap up its campaign in Iran soon and had passed the message on to the United States.
US crude futures fell in early Asian trading hours on Tuesday to their lowest level in more than a week after Trump said a ceasefire had been agreed, relieving worries of supply disruption in the region.
Earlier on Monday, Trump said he would encourage Israel to proceed towards peace after dismissing Iran's attack on an American air base that caused no injuries and thanking Tehran for the early notice of the strikes.
He said Iran fired 14 missiles at the US air base, calling it "a very weak response, which we expected, and have very effectively countered".
Iran's attack came after US bombers dropped 13.6-tonne bunker-buster bombs on Iranian underground nuclear facilities at the weekend, joining Israel's air war.
Much of Tehran's population of 10 million has fled after days of bombing.
The Trump administration maintains its aim was solely to destroy Iran's nuclear program, not to open a wider war.
Trump has cited intelligence reports that Iran was close to building a nuclear weapon, without elaborating.
However, US intelligence agencies said earlier in 2025 they assessed that Iran was not building a nuclear weapon and a source with access to US intelligence reports told Reuters last week that assessment had not changed.
In a social media post on Sunday, Trump spoke of toppling the hardline clerical rulers who have been Washington's principal foes in the Middle East since Iran's 1979 Islamic Revolution.
Israel, however, had made clear that its strikes on Evin prison - a notorious jail for housing political prisoners - and other targets in Tehran were intended to hit the Iranian ruling apparatus broadly, and its ability to sustain power.
