Warships to 'accompany' US-flagged ship

US naval forces have started "accompanying" American-flagged commercial ships passing through the Strait of Hormuz, defence officials have said, after Iran seized a Marshall Islands-flagged vessel this week.

The move means American warships will be in the area and in contact with commercial vessels but not carrying out a full-fledged escort, officials in Washington said.

"US naval forces have begun accompanying US-flagged maritime traffic in the Strait of Hormuz," a defence official said.

The security precaution did not require the deployment of more US ships in the area and naval commanders "will coordinate with shipping industry representatives to ensure accompanying operations are conducted smoothly and efficiently", the official said.

The warships "are close enough to respond if needed" and "in communication" with the cargo ships, but not necessarily within eye sight, a defence official said.

The measure signalled Washington's determination to safeguard commercial shipping in the strategic waterway and to counter any potential attempt by Iran to disrupt American-flagged vessels heading through the narrow shipping channel.

About 30 per cent of all oil traded by sea moves through the chokepoint, or about 17 million barrels a day.

Iranian patrol boats forced a Marshall Islands-flagged ship, the Maersk Tigris, to Iran's Larak Island on Tuesday after firing warning shots. And last week, Iranian forces "harassed" a US-flagged commercial ship in the Gulf.

The incidents prompted Washington to demand Iran uphold freedom of navigation and the provisions of the law of the sea.

Iran has said it seized control of the container ship due to a commercial dispute with Denmark's giant Maersk group, which chartered the vessel to ferry cargo in the region. When it was intercepted, the Tigris was travelling on an international shipping route within Iran's territorial waters.

The US Navy has dispatched a guided-missile destroyer, the USS Farragut, and a P-3 surveillance aircraft to the area to monitor the Maersk Tigris.