DUBAI (REUTERS) - Iranian Revolutionary Guards 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, US officials 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.

Oil prices rose by 60 cents on the news, with Brent futures , the global oil benchmark, rallying as high as US$65.49 before falling back to around US$64.70.

The ship was seized as Teheran inches closer to a final deal with major powers on its nuclear programme. At the same time, it faces escalating tension with Gulf Arab powers over the conflict in Yemen, where jets from a Saudi-led coalition bombed Sanaa airport on Tuesday to block a plane Iran said was carrying aid.

Iranian officials sought to play down the ship seizure, saying it was a civil matter with no military or political dimension, but the Pentagon described the move as an apparent provocation. It was not clear what defence obligation Washington has towards the Marshall Islands, a small Pacific island nation.

Reuters tracking data showed the Maersk Tigris, a 65,000-tonne container ship, anchored off the Iranian coast between the islands of Qeshm and Hormuz. It had been listed as sailing from Saudi Arabia's Red Sea port of Jeddah, bound for the United Arab Emirates port of Jebel Ali in the Gulf.

Iran's state news agency IRNA quoted a source in the Islamic Revolutionary Guards Corps (IRGC) as saying naval units from the Guards force seized the vessel and its 34 crew.

The company managing the vessel told a Danish news channel there were 24 crew members, mostly from eastern Europe and Asia.

The IRGC is Iran's elite military force and operates its own land, naval and air forces under a separate command chain from the regular armed forces.

Mohammad Saidnejad, head of Iran's Ports and Maritime Organisation, said a court in Teheran had given the order to seize the ship.

"A legal complaint from an Iranian private company resulted in the seizure of a Marshall Islands-flagged ship in Iranian waters," Saidnejad was quoted as saying by IRNA.

NUCLEAR ACCORD

The incident was unlikely to derail efforts by the United States and five other powers to secure a final nuclear deal with Iran, even though it was likely to stoke further opposition in the US Congress, risk consultancy Eurasia group said.

Under the proposed nuclear accord Teheran, which denies seeking to build nuclear weapons, would win sanctions relief in return for slashing the number of its uranium enrichment centrifuges and accepting intrusive international inspections.

"We believe the US' and Iran's top priority, by far, is to secure a deal on the nuclear issue, and that the sides will likely resolve this issue quickly," Eurasia's Cliff Kupchan said.

The move may have been a response to the United States placing ships off Yemen to prevent Teheran sending arms to Yemeni Houthi fighters, he added. Iran denies arming the Houthis.

A spokesman for the Singapore-based company that manages the vessel, which usually includes hiring the crew, Rickmers Shipmanagement, said he did not know why Iran had acted.

Spokesman Cor Radings told Danish TV2 news channel that Iranian forces fired warning shots near the container ship and boarded it, and said the company was concerned for the crew.

The vessel had been following a normal commercial route between Saudi Arabia and the United Arab Emirates, he said.

A US government official said the ship was intercepted at 0905 GMT (5.05pm Singapore time). Another US official said when the warning shots were fired, the Maersk Tigris issued a distress call which was received by US forces operating in the region.

The closest US warship was more than 100km away, he said, and the US military instructed destroyer USS Farragut to head towards the cargo ship which was passing through the Strait of Hormuz at the time.

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 US 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 33km across and consists of 3.2km-wide navigable channels for inbound and outbound shipping and a 3.2km-wide buffer zone.