U.S. President Donald Trump on Sunday said the U.S. Navy would “immediately” begin a blockade of ships entering or leaving the Strait of Hormuz, after U.S.-Iran ceasefire talks in Pakistan ended without an agreement.

U.S. Central Command announced that it will blockade all Iranian ports beginning Monday at 10 a.m. EDT, or 5:30 p.m. in Iran.

However, CENTCOM said it would still allow ships traveling between non-Iranian ports to transit the strait. Its announcement was a step down from the president’s earlier threat to blockade the entire strait, and allows traffic to flow in the crucial waterway as long as it avoids Iranian ports.

Trump wants to weaken Iran’s key leverage in the war after demanding that it reopen the strait to all global traffic on the waterway that was responsible for 20% of global oil shipping before fighting began.

Traffic in the Strait has been limited even in the days since the ceasefire. Marine trackers say over 40 commercial ships have crossed since the start of the ceasefire.

A U.S. blockade could further rattle global energy markets.

Oil prices rose in early market trading on Sunday after the blockade announcement. The price of U.S. crude rose 8 per cent to US$104.24 a barrel, and Brent crude oil, the international standard, rose 7 per cent to US$102.29. Brent crude cost roughly US$70 per barrel before the war in late February.

Iran says `if you fight, we will fight’

Iran’s Revolutionary Guard later said the strait remained under Iran’s “full control” and was open for non-military vessels, but military ones would get a “forceful response,” two semi-official Iranian news agencies reported.

During the 21-hour talks, the U.S. military said two destroyers had transited the strait ahead of mine-clearing work, a first since the war began. Iran denied it.

Trump’s plan to use the Navy to block the strait is unrealistic and he will have to concede on some issues with Iran, said Andreas Krieg, a senior lecturer in security studies at Kings College London. “There isn’t any tool in the toolbox in terms of the military lever that he could use to get his way,” Krieg said.

Trump said Tehran’s nuclear ambitions were at the core of the talks’ failure. In comments to Fox News, he again threatened to strike civilian infrastructure.

Iranian Parliament speaker Mohammad Bagher Qalibaf, who led Iran’s side, addressed Trump in a new statement on his return to Iran: “If you fight, we will fight.”

No word on what happens after ceasefire expires

The face-to-face talks that ended early Sunday were the highest-level negotiations between the longtime rivals since the 1979 Islamic Revolution.

Neither indicated what will happen after the ceasefire expires on April 22.

“We need to see an affirmative commitment that they will not seek a nuclear weapon,” said U.S. Vice-President JD Vance, leading the U.S. side.

Iranian negotiators could not agree to all U.S. “red lines,” said a U.S. official who spoke on condition of anonymity because they were not authorized to describe positions on the record. These included Iran never obtaining a nuclear weapon, ending uranium enrichment, dismantling major enrichment facilities and allowing retrieval of its highly enriched uranium, along with opening the Strait of Hormuz and ending funding for Hamas, Hezbollah and Houthi rebels.

Iranian officials said talks fell apart over two or three key issues, blaming what they called U.S. overreach. Qalibaf, who noted progress in negotiations, said it was time for the United States “to decide whether it can gain our trust or not.”

Pakistani Foreign Minister Ishaq Dar said his country will try to facilitate a new dialogue in the coming days. Iran said it was open to continuing dialogue, state-run IRNA news agency reported.

The European Union urged further diplomatic efforts. The foreign minister of Oman, located on the Strait of Hormuz’s southern coast, called for parties to “make painful concessions.” The Kremlin said Russian President Vladimir Putin “emphasized his readiness” to help bring about a diplomatic settlement in a call with Iran’s president.

Iran’s nuclear program is a key sticking point

Iran’s nuclear program was at the center of tensions long before the U.S. and Israel launched the war on Feb. 28. The fighting has killed at least 3,000 people in Iran, 2,055 in Lebanon, 23 in Israel and more than a dozen in Gulf Arab states, and damaged infrastructure in half a dozen countries.

Tehran has long denied seeking nuclear weapons but insists on its right to a civilian nuclear program. The landmark 2015 nuclear deal, which Trump later pulled the U.S. out of, took well over a year of negotiations. Experts say Iran’s stockpile of enriched uranium, though not weapons-grade, is only a short technical step away.

An Iranian diplomatic official, speaking on condition of anonymity because of the sensitivity of closed-door talks, denied that negotiations had failed over Iran’s nuclear ambitions.

Inside Iran, there was new exhaustion and anger after months of unrest that began with nationwide protests against economic issues and then political ones, followed by weeks of sheltering from U.S. and Israeli bombardment.

“We have never sought war. But if they try to win what they failed to win on the battlefield through talks, that’s absolutely unacceptable,” Mohammad Bagher Karami said in Tehran.

Elsewhere in the region, airstrikes calmed over the past day except in Lebanon.

More questions as Israel presses ahead in Lebanon

Iran’s 10-point proposal for the talks called for a halt to Israeli strikes on the Iranian-backed Hezbollah in Lebanon. Israel has said the ceasefire did not apply there, but Iran and Pakistan said it did.

Israeli Prime Minister Benjamin Netanyahu visited parts of southern Lebanon under Israeli control on Sunday, for the first time since the current fighting. Attacks on southern Lebanon have intensified alongside the ground invasion renewed after Hezbollah launched rockets toward Israel in the war’s opening days.

Negotiations between Israel and Lebanon are expected to begin Tuesday in Washington after Israel’s surprise announcement authorizing talks despite their lack of official relations. Israel wants Lebanon to assume responsibility for disarming Hezbollah, but the militant group has survived efforts to curb its strength for decades.

The day the Iran ceasefire deal was announced, Israel pounded Beirut with airstrikes, killing more than 300 people, according to the Health Ministry.

Lebanon’s state-run National News Agency reported six people were killed Sunday in Maaroub village near the coastal city of Tyre.