Amid the apparent collapse of the U.S.-Iran negotiations, tensions escalated with President Trump’s announcement Monday that the naval blockade with which he had threatened Iran had begun.

“Right now there’s no fighting. Right now we have a blockade,” he said, talking to reporters at the White House. Iran is doing “absolutely no business. And we’re going to keep it that way very easily.”

The announcement comes even as an unnamed U.S. official said engagement with Iran is ongoing, claiming that the two parties are moving forward to come to an agreement.

Meanwhile, Iran threatened retaliation against Gulf ports in response to the blockade.

Oil prices leaped past $100 a barrel, with no sign of the Strait of Hormuz, a vital waterway for shipping about a fifth of the world’s oil, reopening any time soon.

The fragile ceasefire, with one week left to go, is endangered by the rising hostilities. Trump said that Iran has refused to accede to U.S. demands to give up its nuclear program and its entire stockpile of enriched uranium, but that he would never let the authoritarian regime achieve its goal of obtaining a nuclear weapon.

“Iran will not have a nuclear weapon,” he declared at the White House. “We can’t let a country blackmail or extort the world.”

Since the U.S.-Israeli joint campaign against Iran began on Feb. 28, Iran has shut down the Strait of Hormuz to all ships except those belonging to friendly countries, unless they are willing to pay exorbitant fees and subject themselves to Iranian control.

The oil market responded accordingly, and now Trump has implemented a blockade on Iranian exports to pressure the Iranian regime. He also warned Iran that the U.S. will destroy Iranian vessels attempting to approach the blockade.

“Warning: If any of these ships ⁠come anywhere close to our BLOCKADE, they will be immediately ELIMINATED, using the same system of ​kill that we use against the drug dealers on boats at Sea. It is quick and ​brutal,” Trump wrote.

U.S. Central Command confirmed that the blockade had been implemented but that it would not block ships passing through Hormuz.

“The blockade will not impede neutral transit passage through the Strait of Hormuz to or from non-Iranian destinations,” CENTCOM said.