LONDON — Explosions crippled two oil tankers in the Gulf of Oman on Thursday in what the United States called “unprovoked attacks” by Iran, raising alarms about immediate security and potential military conflict in a vital passageway for a third of the world’s petroleum.

Iran called the accusations part of a campaign of American disinformation and “warmongering.”

The explosions forced the crews of both vessels to evacuate and left at least one ablaze, and hours later the causes were still under investigation. Yet the backdrop of steeply rising threats between President Trump and Iranian leaders gave the stricken ships a grave significance even before the facts became clear.

[Read: Operator says a ‘flying object’ struck one tanker, not a mine]

By afternoon, Secretary of State Mike Pompeo said that American intelligence agencies had concluded Tehran was behind the disabling of both ships. He pointed to the weapons used, the expertise and resources required and the similarity to other recent attacks attributed to Iran.

The most compelling evidence to support Mr. Pompeo’s claim was video footage released Thursday night by the United States Central Command. A military spokesman, Capt. Bill Urban, said the video showed an Islamic Revolutionary Guards Corps patrol boat pulling up alongside the Kokuka Courageous, one of the stricken ships, several hours after the initial explosion, and removing an unexploded limpet mine in broad daylight.