WASHINGTON (AFP/REUTERS) - Iranian Revolutionary Guard forces fired warning shots at a Singapore-flagged commercial ship in the Gulf on Thursday, before vessels from the United Arab Emirates came to the ship's aid, US officials said.

The incident reflects rising tensions in the Gulf, fuelled in part by the conflict in Yemen that pits Iranian-backed Shiite rebels against pro-government forces supported by a Saudi-led coalition of Sunni Arab states.

The Iranian patrol craft fired across the bow of the Alpine Eternity in international waters and the commercial ship then headed towards the UAE, two US defence officials, who spoke on condition of anonymity, told AFP.

The Alpine Eternity issued a radio call for help and the Iranian boats fired a second wave of warning shots, the officials said.

UAE authorities heard the radio call and deployed coast guard boats in response. The Iranian boats then departed the area, the US officials said, adding that there were no Americans aboard the commercial ship.

A US naval ship was about 32km away at the time and "monitored the radio traffic" but received no request for assistance, Pentagon spokesman Colonel Steven Warren told reporters.

Singapore's Maritime and Port Authority said no Singaporeans were on board and that it "verified with the ship manager of Alpine Eternity that the crew members are safe and the vessel is currently in UAE waters."

The tanker's owner, South Maritime Pte Ltd, said in a statement that the ships first fired warning shots and then directly fired on the vessel after it ignored an order to stop. "No serious damage was sustained by the vessel and none of the 23 crew members were injured," the statement said. The owner said the vessel safely reached the port of Jebel Ali.

The Pentagon acknowledged that it was the third incident in a few weeks involving Iranian forces confronting commercial vessels in or near the strategic Strait of Hormuz.

"This is something that we continue to watch, that we're concerned with," he said.

White House Deputy National Security Adviser Ben Rhodes said: "This is exactly the type of challenge that many of the (Gulf) partners are focused on."

Iran recently seized a Marshall Islands- flagged vessel in the Gulf, harassed a US- flagged ship and warned the United States not to interfere with an Iranian aid ship headed for Yemen. The Marshall Islands- flagged vessel, the Maersk Tigris, was eventually released after Iranian officials said a commercial dispute had been settled.

- 'Robust' US presence -

Following the seizure of the Maersk Tigris, the US Navy recently provided security for American- flagged commercial ships crossing through the Strait of Hormuz. But US commanders called it off after about a week, saying tensions had appeared to ease.

Asked if the beefed up security for the vital waterway was cancelled too soon, Warren said: "We're always in a position to reevaluate in the future."

Twelve US naval ships, including an aircraft carrier, the USS Theodore Roosevelt, are currently in the region but no additional vessels were due to deploy to the area as a result of Thursday's incident.

"We maintain a robust presence in the Persian Gulf," he said.

Another possible maritime showdown emerged this week. The United States demanded that an Iranian aid ship bound for Yemen change course and head towards Djibouti, where the United Nations is overseeing humanitarian efforts for the Yemen conflict.

But a senior Iranian commander rejected the request and warned the Americans that "a fire might start" if there were attempts to block Teheran's aid efforts.

Although Iran has said it would provide a naval escort for the aid ship, so far the vessel has no warships guarding it. The Iranian cargo ship is currently heading south-west off the coast of Oman, Warren said.

The ship in Thursday's incident, the Alpine Eternity, is an oil-chemical tanker that last stopped at a port in Bahrain, according to the marinetraffic.com shipping website.

The vessel is listed as part of the fleet of Transpetrol, a commercial shipping firm that transports oil and gas, with offices in Belgium, Bermuda, Norway and Switzerland.

Shipping industry officials said they were bracing for the likelihood of even more tensions at sea, which could lead to a spike in shipping costs.

"The pattern looks like the Revolutionary Guards are using a commercial pretext to intervene in the incidents to date," said one shipping underwriter. "This could start to impact upon (insurance) rates."