President Trump threatened renewed war with Iran on Sunday night in his latest ultimatum to the country, which has so far resisted U.S. demands to largely shut down its nuclear program.

Mr. Trump said in a social media post that Iran had to move fast “or there won’t be anything left,” adding “the Clock is Ticking.” He did not set a deadline and over the last two months, Mr. Trump has made similar threats to Iran without following through.

Negotiations between the United States and Iran have been stalled for weeks. Mr. Trump has repeatedly warned that he could soon order a renewed assault on Iran unless its leaders made concessions in the talks.

Iran has repeatedly rebuffed U.S. terms for a deal to curb uranium enrichment and end attempts to blockade the strategic Strait of Hormuz, a critical waterway for shipping oil and gas. The turmoil over the strait has roiled markets and sent the price of oil soaring.

On Monday, Iran said it had handed yet another counterproposal to the United States in the negotiations. The Iranian foreign ministry said the talks were still continuing through Pakistan, which has been mediating between the two sides.

Asked about Mr. Trump’s threats, Esmaeil Baghaei, the Iranian foreign ministry spokesman told reporters: “Don’t worry, we know very well how to respond.” He did not elaborate further, although Iranian officials have repeatedly threatened fierce retaliation if attacked again by the United States and Israel.

The Pentagon is planning for the possibility that Operation Epic Fury — which was paused when the president declared a cease-fire last month — will pick up again in the coming days. Benjamin Netanyahu, Israel’s prime minister, said he spoke with Mr. Trump over the weekend.

“We are closely watching Iran. I will speak today, as I do every few days, with our friend President Trump,” Mr. Netanyahu said on Sunday. “I will certainly hear impressions from his trip to China, perhaps other things as well. Of course there are many possibilities. We are prepared for any scenario.”

Two Middle East officials, who spoke on condition of anonymity to discuss operational matters, said last week that the United States and Israel are engaged in intense preparations — the largest since the cease-fire took effect — for the possible resumption of attacks against Iran as early as this week.

The war began in late February with a joint U.S.-Israeli attack on Iran that drew much of the Middle East into the conflict.

But analysts say the United States has faced a tough dilemma since a cease-fire was declared last month. U.S. and Israeli war planes could again start attacking Iran from the air, but many military analysts say bombing alone is unlikely to force Iran to agree to American demands.

The two countries could launch a special forces operation to try to seize Iran’s stockpile of highly enriched uranium, which the U.S. and Israel fear could be used to build a nuclear weapon. But such a raid could risk the lives of American soldiers, further straining domestic U.S. support for an unpopular war.

For the last month, Mr. Trump has instead opted for a pressure campaign — so far unsuccessful — to force Iran to reopen the Strait of Hormuz.

The United States briefly launched what Mr. Trump called “Project Freedom” to help escort ships trapped in the waterway to safety. Just a day later, the initiative was suspended to allow for further negotiations with Iran.