The U.S. Armed Forces said that two U.S. Navy guided-missile destroyers entered the Persian Gulf to break the Iranian blockade, and that two American ships crossed the Strait of Hormuz after Iran said it had prevented a U.S. warship from entering the Persian Gulf.

U.S. Central Command (CENTCOM) said its forces are supporting President Donald Trump’s “Freedom” project, aimed at “bringing out commercial vessels” stranded in the Persian Gulf because of the U.S.-Israeli war with Iran, and are tightening the blockade of Iranian ports.

The intervention appears to have raised the risk of a direct confrontation between the United States and Iran on the waterway through which normally a fifth of the world’s seaborne oil and gas supplies are transported, but which has been blocked for two months as a result of the war.

CENTCOM said that two U.S.-flagged commercial vessels crossed the strait while American destroyers were operating in the Persian Gulf.

“The U.S. military is actively facilitating efforts to restore transit for commercial shipping,” U.S. Central Command added.

Earlier on Monday, Iran said it had forced a U.S. warship to turn back from the Strait of Hormuz, and CENTCOM denied Tehran’s claim that the ship had been hit by missiles. A senior Iranian official told Reuters that Iran fired a warning shot and that it was unclear whether the warship had been damaged.

Oil prices jumped 5% on reports that the warship had been turned back, but by 12:20 GMT they were up about 2.5%.