Dubai: Saudi-owned Al Arabiya television said an Iranian "force" seized a US cargo ship after opening fire on it in the Gulf on Tuesday and directed it to the Iranian port of Bandar Abbas.

A Pentagon spokesman said Iranian forces had boarded a Marshall Island-flagged vessel, the MV Maersk Tigris, in the Gulf. He said the boarding occurred after Iranian patrol boats fired shots across the vessel's bow and ordered it deeper into Iranian waters.

The ship had no US citizens aboard, the spokesman said, contradicting Al-Arabiya's report which said there were 34 US sailors on board. He said it was travelling through the Strait of Hormuz when the incident occurred.

There was no immediate word from Iranian officials.

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 (21 miles) across and consists of 2-mile wide navigable channels for inbound and outbound shipping and a 2-mile-wide buffer zone.