WASHINGTON — U.S. President Donald Trump threatened Iran with intensified military action and said he’s unsure whether a diplomatic agreement can be reached, with the two sides at loggerheads on how to end the near monthlong war.

Iran had “better get serious soon, before it is too late, because once that happens, there is NO TURNING BACK, and it won’t be pretty!” Trump posted on social media Thursday, after Tehran rejected an initial U.S. ceasefire proposal via intermediaries overnight.

Iran has “great negotiators, and they are begging to work out a deal,” the president said later during a Cabinet meeting at the White House. “I don’t know if we’ll be able to do that.”

The Islamic Republic is waiting for a response to its rejection of a U.S. 15-point plan to end the war, the semi-official Tasnim news agency reported. Tehran has a string of its own conditions, Tasnim said, one of which is a guarantee that the United States and Israel won’t resume their attacks.

Oil prices surged on the developments, with optimism fading of a quick resolution to the impasse. Brent crude climbed 6% to more than $108 a barrel, while stocks and bonds fell worldwide.

The U.S. has compiled a list of a dozen demands — alongside three points Iran would get in return — that have been delivered to Tehran by mediators in Pakistan, according to people familiar with the matter. U.S. envoy Steve Witkoff confirmed the 15-point proposal Thursday, without giving details, and said it had led to “strong and positive messaging and talks.”

Iran’s conditions to agree to a ceasefire include the payment of reparations for war damages and recognition of Iran’s authority over the Strait of Hormuz, Tasnim said, citing an informed source.

Trump is under pressure to persuade Tehran to reopen the critical waterway for oil and gas flows, a step needed to arrest a global supply shock. On Thursday, he said Iran allowed 10 boats of oil to sail through the strait as a gesture of goodwill.

Iran is also calling for an end to the war on all fronts, Tasnim reported, a likely reference to Israel’s parallel war against the Tehran-backed Hezbollah militant group in Lebanon.

Trump’s threat of escalation came after the White House ordered more troops to the region, with some set to arrive before week’s end. On Thursday, he repeated an earlier timeline of four to six weeks for military operations and said the U.S. war effort is “ahead of schedule.”

Both sides kept up their air attacks Thursday. The Israel Defense Forces completed a wave of strikes in the central Iranian city of Isfahan, while Iran’s state TV said the country had begun another wave of missile strikes against Israel. Two people were killed after debris from an intercepted missile fell in Abu Dhabi.

Iran is looking to formalize a transit fee for the Strait of Hormuz, with lawmakers working on a draft bill to impose a toll in exchange for providing security to ships, according to the Fars news agency. The strait is a conduit for about a fifth of the world’s oil and liquefied natural gas.

The Islamic Republic is still able to export its own crude from the strait, likely earning hundreds of millions of dollars of extra income.

Trump had set a deadline for Iran to negotiate an agreement to end the war by the end of the week, though the ongoing impasse has raised questions about the likelihood for a deal in that time frame. Asked for an update Thursday, Trump would only say “we’ll see.”

The conflict has led to surging fuel and fertilizer prices, and sparked fears of an inflation crisis and worldwide food shortages.

The OECD on Thursday sharply increased its inflation forecasts for major economies and now sees the average rate for the Group of 20 this year jumping to 4% — with an even higher pace in the U.S. — rather than the 2.8% it predicted in December.

Trump has publicly signaled any peace agreement would have to include a prohibition on Iran ever obtaining a nuclear weapon or enriching radioactive material for civilian purposes.

The U.S. plan also stipulates that the Islamic Republic use a reduced missile arsenal in self-defense only, according to people familiar with the matter. Iran would receive certain concessions in return, including sanctions relief.

It’s still unclear who the U.S. is negotiating with since several top Iranian government and military officials have been killed. On Monday, Axios identified Mohammad-Bagher Ghalibaf, Iran’s speaker of parliament, as the likely front man for talks, though he denied negotiations have taken place.

Israeli officials have shown no inclination to end the fighting.

“At this stage we are still at war, and when it might end, no one knows,” Israeli Energy Minister Eli Cohen told radio station Galey Israel. The Israeli military and “defense establishment have the stamina and determination to see this historic campaign, in whose midst we find ourselves, to its conclusion.”

Gulf states such as Saudi Arabia and the UAE are considering joining the war against Tehran, several people with knowledge of the situation said this week.

“We can’t let Iran hold the U.S., the United Arab Emirates and the global economy hostage,” UAE Ambassador to the United States Yousef Al Otaiba wrote in a Wall Street Journal op-ed. “A simple cease-fire isn’t enough. We need a conclusive outcome that addresses Iran’s full range of threats.”

More than 4,500 people have been killed in the conflict, according to governments and nongovernment agencies. Around three-quarters of the fatalities have been in Iran, while almost 1,100 people have died in Lebanon, where Israel is fighting a parallel war against Iran-backed Hezbollah militants and has displaced more than a million people. Dozens have been killed in Israel and Arab Gulf states.

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(With assistance from Dana Khraiche, Dan Williams, Eltaf Najafizada and Meghashyam Mali.)

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