WASHINGTON • President Donald Trump warned Iran on Thursday that it must make a deal over its nuclear program or “really bad things” will happen.

The president also appeared to set a deadline of no more than 10 to 15 days before the U.S. might take action.

Amid a massive U.S. military buildup in the Middle East, Trump said negotiations with Iran to end the tense standoff are going well but insisted Tehran has to reach a “meaningful” agreement.

“Otherwise bad things happen,” Trump told the first meeting of his Board of Peace in Washington.

Trump spoke of the U.S. airstrikes carried out in June, saying Iran’s nuclear potential had been “decimated,” adding “we may have to take it a step further or we may not.”

“You’ll be finding out over the next probably 10 days,” he said. Asked later to elaborate, he told reporters aboard Air Force One: “I would think that would be enough time — 10, 15 days, pretty much maximum.”

He declined to be specific, except to again warn of “really bad things” and to insist that Iran would have to make a deal one way or another.

Against this backdrop, oil prices went up and a Russian corvette warship on Thursday joined planned Iranian naval drills in the Gulf of Oman, a vital sea route for global energy.

Iranian and U.S. negotiators met on Tuesday and Iranian Foreign Minister Abbas Araqchi said they agreed on “guiding principles.” White House press secretary Karoline Leavitt said on Wednesday, however, that the two sides remained apart on some issues.

Trump said “good talks are being had,” and a senior U.S. official said Iran would make a written proposal on how to address U.S. concerns.

Trump called on Tehran to join the U.S. on the “path to peace.”

“They can’t have a nuclear weapon, it’s very simple,” he said. “You can’t have peace in the Middle East if they have a nuclear weapon.”

Iran has resisted making major concessions on its nuclear program, though the country insisted it is for peaceful purposes. The U.S. and Israel in the past have accused Tehran of trying to develop a nuclear bomb.

Trump’s latest comments came as he hosted world leaders at the launch of his Board of Peace. He proposed the body in September when he announced his plan to end Israel’s war against Hamas in Gaza.

Earlier on Thursday, Russia warned against an “unprecedented escalation of tension” around Iran and urged restraint amid the U.S. military buildup in the region, which a senior American official said should be complete by mid-March. 

Trump has sent aircraft carriers, warships and jets to the region. The United States and Israel bombed Iran’s nuclear facilities and some military sites last June.

U.S. Secretary of State Marco Rubio will meet Israeli Prime Minister Benjamin Netanyahu to discuss Iran on Feb. 28, the senior U.S. official said.  

Washington wants Iran to entirely give up uranium enrichment, a process used to create fuel for atomic power plants but that can also provide material for a warhead. 

The U.S. and ally Israel also want Iran to give up long-range ballistic missiles, stop supporting proxy groups around the Middle East and stop using force to quell internal protests. 

Iran has refused to discuss issues beyond the atomic file, calling efforts to limit its missile arsenal a red line. 

Satellite pictures have tracked both Iranian work to repair and fortify sites since last summer, showing work at both nuclear and missile sites, as well as preparations at U.S. bases across the Middle East over the past month. 

Iran’s joint exercise with Russia came days into an extended series of Iranian naval drills in the Gulf of Oman, with Iranian state television showing special forces units deployed on helicopters and ships. 

In a sign of growing concern over the increased tensions, Poland on Thursday became the latest European country to urge its citizens to leave Iran, with Prime Minister Donald Tusk saying Poles may only have hours to evacuate. 

Trump began threatening strikes on Iran in January as Iranian authorities crushed widespread protests with deadly violence that left thousands dead across the country.