A vessel contracted by the Navy in the Persian Gulf fired warning shots Thursday on two fast boats believed to be of Iranian origin.

In a story first reported by FOX News, Navy officials said the Westward Venture fired upon two boats about 50 miles off the coast of Iran.

U.S. Navy Fifth Fleet spokeswoman Cmdr. Lydia Robertson said the boats were of unknown origin, but other Navy officials told FOX News that the markings of the boats and their behavior led them to believe they were Iranian and typical of those used by the Iranian Revolutionary Guard.

The Iranian Navy countered those claims Friday, saying on Iranian television station Al-Alam that they didn't have a confrontation with a U.S. ship and that the Westward Venture may have fired on a non-Iranian vessel, Reuters reported.

The approximately 1,000-foot-long Westward Venture is contracted by the U.S. Military Sealift Command to carry military cargo to Kuwait. It fired upon the fast boats while traveling north in international waters. Defense officials told FOX News it did so after attempts to get the boats' attention failed.

A Navy security team of 12, armed with M-16 rifles and .50-caliber machine guns, was onboard the Westward Venture at the time the warning shots were fired.

The incident, which lasted for about 15 minutes, began when the Westward Venture attempted to make bridge-to-bridge contact to warn the fast boats that they were too close.

The Military Sealift Command vessel then blew its whistle and fired flares before finally firing warning shots with the machine guns and M-16s when the boats came within 100 yards of the cargo ship.

Bridge-to-bridge communication was established after the shots were fired, with someone claiming to be the Iranian Coast Guard contacting the Westward Venture.

"It is not clear if this was one of the small boats or a separate boat," Robertson said.

The Iranian Revolutionary Guard is suspected of being involved in this incident, officials told FOX News.

Defense officials said that typically the guard behaves in a very unprofessional and aggressive manner, using unmarked boats and uniforms, unlike the Iranian Navy.

Oil prices rose sharply Friday on news of the confrontation, on concerns that a conflict between U.S. and Iranian forces could cut oil supplies from the Persian Gulf region. The cost for a barrel of light, sweet crude oil for June delivery rose $3.08 to $119.14 on the New York Mercantile Exchange after earlier rising as high as $119.50.

Prices were already up before the report on news of a pipeline attack in Nigeria and a looming refinery strike in Scotland.

The incident follows two other encounters between U.S. and Iranian ships. Five small Iranian boats swarmed three U.S. warships in the Gulf's narrow Strait of Hormuz in early January. The U.S. commanders did not fire any warning shots despite hearing a strange radio call saying the boats would explode, and the Iranians eventually retreated.

In mid-December, a U.S. ship fired a warning shot at a small Iranian boat that came too close in the Strait of Hormuz, causing the Iranians to pull back.

Thursday's incident took place in the central Persian Gulf, not the Strait of Hormuz, officials said.

FOX News' Justin Fishel and Jennifer Griffin, and the Associated Press contributed to this report.