Iranian leaders, in response, have threatened to block the Strait of Hormuz, a potential Persian Gulf chokepoint. After complying with the nuclear pact for a year even after the American withdrawal, Iran has also raised the possibility of breaching the accord by taking initial steps to expand its supply of enriched uranium.
Some Iranian allies around the region have stepped up their attacks on allies of Washington, fueling fears of a wider conflict. The Houthi faction in Yemen, which is backed by Iran, has launched attacks on Saudi oil pipelines and other targets, and this week a Houthi missile hit the arrivals hall of a Saudi airport, injuring 26 people, according to Saudi news reports.
The attacks in May on the four tankers near Fujairah were relatively minor, causing only limited damage to the hulls. An international investigation presented to the United Nations later concluded that the damage was done by divers deployed from small “fast boats” who had placed limpet mines against the hulls.
On a visit to the United Arab Emirates about two weeks ago, John R. Bolton, President Trump’s national security adviser, said Iran was “almost certainly” responsible. A few days later, Mr. Pompeo called the attacks “efforts by the Iranians to raise the price of crude oil.”
The explosions that disabled the tankers in the Gulf of Oman on Thursday were far more severe.
Both took place around dawn, with distress calls at 6:12 a.m. and 7 a.m., according to a statement from the United States Fifth Fleet, which said the U.S.S. Bainbridge, a guided missile destroyer, was “rendering assistance.”
A Norwegian company that owns one of the ships, the Front Altair, confirmed that it was on fire. The crews of both vessels — about 23 in one and 21 in the other — were evacuated in lifeboats.
The owners and operators of both vessels described the incidents as deliberate attacks.
The Front Altair, registered in the Marshall Islands, was chartered by the CPC Corporation, the Taiwanese oil company, to carry naphtha, a petroleum product, from the Emirati port of Ruwais to Kaohsiung, Taiwan.