DUBAI, April 11 (Reuters) - Three boats approached a U.S. Navy vessel at high speed in Gulf waters on Thursday but stopped after it fired a flare, the U.S. Fifth Fleet said.

The incident followed a confrontation in January, when the United States said Iranian boats aggressively approached three U.S. Naval ships and warned that they would explode. Iran said that was a routine contact.

“The USS Typhoon, a patrol craft, was approached by three high speed boats of unknown origin,” a spokeswoman for the Bahrain-based Fifth Fleet told Reuters on Friday.

She said the U.S. vessel, which was sailing from the central to the northern part of the Gulf, tried to communicate with the boats by routine methods but decided to fire the flare after receiving no response.

“The small boats then stopped and fell out of the visibility range and nothing further happened,” she said, adding that the boats bore no flag or markings identifying their origins or purpose.

The Gulf is vital to world oil shipments, with around 17 million barrels per day of crude oil, over a third of total global shipments, passing through the waterway.

A U.S. Defense Department official in Washington said Thursday’s incident was not viewed as serious.

“They (U.S. Navy officials) did not view it as a serious or credible threat,” the official said.

Tensions between Washington and Tehran are already high over Iran’s nuclear programme. Iran says its nuclear programme is aimed at electricity generation but the United States suspects it wants to develop atomic bombs. (Reporting by Lin Noueihed; Editing by Giles Elgood)

Editorial Note - USS Typhoon (PC-5) is the fifth United States Navy Cyclone class Patrol (coastal) ship. Typhoon was laid down May 15, 1992 at Bollinger Machine, in Lockport, Louisiana and launched March 3, 1993. She was commissioned February 12, 1994 in Tampa, Florida. As of 2008, Typhoon operates in the Persian Gulf, stationed in Manama, Bahrain since 2004 and is manned by one of 13 rotating 30 person US Navy crews and a US Coast Guard Law Enforcement Detachment (LEDET) that performs ship boardings.