WASHINGTON (AP) — President Donald Trump said he is holding off on a military strike on Iran planned for Tuesday because "serious negotiations" are underway.

Trump made the announcement in a social media post on Monday after threatening over the weekend that time was running out for Iran to strike a deal or fighting would renew. Last week, he said a fragile ceasefire was on "life support," and U.S. forces exchanged fire with Iranian forces.

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The president, who had not previously disclosed that he was planning a strike for Tuesday, did not offer details about the planned attack in his Monday post but said he instructed the U.S. military "to be prepared to go forward with a full, large scale assault of Iran, on a moment's notice, in the event that an acceptable Deal is not reached."

Trump has been threatening for weeks that the ceasefire struck in mid-April could end if Iran did not strike a deal, with shifting parameters for striking such an agreement. Over the weekend he warned, "For Iran, the Clock is Ticking, and they better get moving, FAST, or there won't be anything left of them."

Trump said he was calling off the planned strike at the request of allies in the Middle East, including the leaders of Qatar, Saudi Arabia and the United Arab Emirates.

The president has repeatedly set deadlines for Tehran and then backed off. But he's also in the past indicated he would hold off on military action to allow talks to continue — only to turn around and launch strikes. That's what happened at the war's outset, when he ordered strikes shortly after indicating he would let talks play out.

Trump in recent days has also spoken with Israeli Prime Minister Benjamin Netanyahu and Chinese President Xi Jinping about the Iran war.

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Earlier Monday, Turkey's Foreign Minister Hakan Fidan said the immediate concern of the negotiations between the U.S. and Iran was keeping the Strait Of Hormuz open, but Iran's nuclear program remained a central issue.

Speaking during a joint news conference with his German counterpart in Berlin, Fidan said much of Iran's enriched uranium that could potentially be used for a nuclear weapon was buried under collapsed tunnels following attacks in June that the U.S. launched with Israel. The U.S. has said it is closely monitoring any movements around the stockpile.

"At present, there isn't a situation that poses a real threat," Fidan said. "But for this to continue, the parties must reach and conclude a nuclear negotiation among themselves."

The Turkish minister said he believes Iran is not opposed in principle to complying with nuclear conditions, but added: "the question is what will be given in return, in what order, and under what conditions."

With talks at a standstill last week, Iran's foreign minister said Friday that a lack of trust was the biggest impediment to negotiations.

Iran, which says its nuclear program is for peaceful purposes, was said to have included some nuclear concessions in its latest proposal to end the war. But Trump dismissed the proposal as "garbage."

Associated Press writer Suzan Fraser in Ankara, Turkey, contributed to this report.