Andreas Krieg, a senior lecturer in security studies at Kings College London, said Sunday that Trump’s plan to use the U.S. Navy to block the Strait of Hormuz is unrealistic.

“We should bear in mind that the Americans have a much lower threshold of pain than the Iranians,” Krieg said. “The Iranians, whatever happens, can sustain this for far longer than the world economy, far longer the Gulf states, far longer then the Americans.”

Krieg said Trump doesn’t have “any good options” and that he will have to concede on some issues.

“There isn’t any tool in the toolbox in terms of the military lever that he could use to get his way,” he said.

Earlier, President Donald Trump said the U.S. Navy would swiftly 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.

CENTCOM said the blockade will be “enforced impartially against vessels of all nations.” It said it would still allow ships traveling between non-Iranian ports to transit the Strait of Hormuz.

Earlier in the day, the United States and Iran ended 21 hours of face-to-face talks in Islamabad without reaching a deal, leaving the fate of the fragile, two-week ceasefire still unclear.

The war that has killed thousands of people and shaken global markets has entered its seventh week.