An oil tanker that’s part of a U.S.-military fuel procurement program was hailed by small armed ships in the Strait of Hormuz off Iran’s coast, amid heightened tensions between the two countries. 

Iran’s Islamic Revolutionary Guard Corps “harassed” the U.S.-flagged Stena Imperative as it was transiting through the vital waterway, U.S. Central Command said in a statement on February 3.

Two IRGC boats and a drone approached the U.S. vessel “at high speeds and threatened to board and seize the tanker” before a U.S. guided-missile destroyer in the area escorted the U.S. vessel to safety, CentCom said.

Iranian media said the country’s naval forces warned a vessel to leave Iranian territorial waters after failing to produce the necessary legal documents. The vessel departed immediately after the warning, Iran’s semi-official Fars News said.

The incident, which was earlier reported by the U.K.’s naval liaison in the region, occurred in a part of the inbound maritime corridor into the Strait of Hormuz, the narrow chokepoint that allows ships to enter and exit the Persian Gulf and accounts for about a quarter of the world’s seaborne oil trade. 

President Donald Trump last week threatened a fresh attack on Iran, heightening tensions in the region. He has since said talks between the two countries over a new nuclear deal could take place in the coming days.

The ship is part of the U.S. Tanker Security Program, which aims to ensure that the Department of Defense has access to a fleet of American-flagged fuel tankers at all times.

Iran has in the past both harassed and seized ships sailing near to its shores. Last year, it diverted a ship called the Talara to its waters, before unloading its cargo and releasing it.

Late last week, Greece, the world’s largest tanker-owning nation advised its ships to exercise caution while transiting the strait.