WASHINGTON (Reuters) - A cargo ship hired by the U.S. military fired warning shots at approaching boats in the Gulf, the U.S. Navy said on Friday, underscoring tension in the region as the Pentagon sharpened its warnings to Iran.

A cargo ship contracted by the U.S Military Sealift Command has fired at least one shot toward an Iranian boat, a U.S. defense official said on Friday. REUTERS/Graphics

According to American defense officials, the Westward Venture cargo ship chartered by the U.S. Defense Department was traveling in international waters when two unidentified small boats approached on Thursday.

After the boats failed to respond to radio queries and a warning flare, the cargo ship’s security team fired “a few bursts” of machine gun and rifle warning shots, according to Cmdr. Lydia Robertson, a spokeswoman for the U.S. Navy’s Bahrain-based Fifth Fleet.

“The small boats left the area a short time later,” she said by telephone. “They were able to avoid a serious incident by following the procedures that we use.”

The news helped push oil prices up more than $3 to $119.50 a barrel -- within striking distance of the record $119.90 hit earlier this week -- as traders worried escalating tensions in the region could eventually disrupt crude shipments.

U.S. defense officials, speaking only on condition of anonymity, first said they suspected the boats were Iranian.

But a Fifth Fleet spokeswoman quickly backed away from that charge.

“We cannot speculate on who they are. We just don’t know. We have no proof of who they were,” said Lt. Stephanie Murdoch, another spokeswoman for the Fifth Fleet.

In Tehran, an Iranian navy source denied that any confrontation had occurred with a U.S. ship in the Gulf. But the source, quoted by a journalist for Iran’s state-owned Arabic Al-Alam TV channel, said any shooting that may have occurred could have targeted a non-Iranian vessel.

SIMMERING TENSION

Relations between Washington and Tehran are tense over Iran’s nuclear program and who is to blame for ongoing violence in Iraq. Hostile rhetoric and close encounters in the Gulf have fueled speculation that the United States may be planning some sort of military action against Tehran.

U.S. charges of Iranian involvement in threats against its ships have also contributed to the tension.

In January, for example, the United States said five small Iranian speed boats aggressively approached three U.S. Navy ships in the Strait of Hormuz, a critical crude oil shipping route. During the confrontation, a radio message was received warning the U.S. ships they could explode within minutes.

But Iran said its boats were simply trying to identify the U.S. vessels and maritime experts said the threatening message may have come not from the Iranian boats but from a radio heckler known as “the Filipino monkey.”

In March, another U.S. military-chartered ship preparing to cross the Suez Canal fired warning shots at a small boat, killing an Egyptian on board.

The latest incident came as America’s top military officer charged Iran with increasing support for Iraqi militias and warned that the United States had military options to force Tehran to stop.

“When I say I don’t want to take any military options off the table, that certainly more than implies that we have military options,” said Adm. Michael Mullen, chairman of the U.S. Joint Chiefs of Staff. “That kind of planning activity has been going on for a long time. I think it will go on for some time into the future,” he told reporters

While U.S. officials repeatedly deny plans to strike Iran, they have not closed the door completely on military action.

“Another war in the Middle East is the last thing we need and, in fact, I believe it would be disastrous on a number of levels,” Defense Secretary Robert Gates said this week.

“But the military option must be kept on the table given the destabilizing policies of the regime and the risks inherent in a future Iranian nuclear threat -- either directly or through proliferation.”