Iranian forces boarded a Marshall Islands-flagged cargo ship in the Gulf on Tuesday after patrol boats fired warning shots across its bow and ordered it deeper into Iranian waters, the Pentagon said.

US planes and a destroyer were monitoring the situation after the vessel, the MV Maersk Tigris, made a distress call in the Strait of Hormuz, one of the world's most important oil shipping channels.

There was no immediate word from Iranian officials.

Saudi-owned Al Arabiya television earlier said an Iranian force fired on and seized a US cargo ship with 34 US soldiers on board, and directed it to the Iranian port of Bandar Abbas. But the Pentagon spokesman said there were no US citizens on board the ship.

Reuters tracking data showed the Maersk, a 65,000-tonne container ship, off the Iranian coast between the islands of Qeshm and Hormuz. It was listed as sailing from the Saudi port of Jeddah, bound for the United Arab Emirates port of Jebel Ali.

A US government official said the ship was intercepted by the Naval force of Iran's Islamic Revolutionary Guard Corps (IRGC) at 0905 GMT.

The Pentagon spokesman said the incident occurred when the Maersk Tigris was passing through the Strait of Hormuz. Some 17 million barrels per day (bpd), or about 30 per cent of all seaborne-traded oil, passed through the channel in 2013, according to the U.S. Energy Information Administration (EIA).

Iran has in the past sometimes threatened to block the strait to advance its opposition to sanctions imposed over its nuclear programme.

The channel is a narrow strip of water separating Oman and Iran. It connects the biggest Gulf oil producers, such as Saudi Arabia, with the Gulf of Oman and the Arabian Sea.

At its narrowest point, the strait is 33 km across and consists of 2-mile wide navigable channels for inbound and outbound shipping and a two-mile-wide buffer zone.-Reuters