US Navy insists it had no option after killing 'fisherman' in Gulf

Tensions and patrols rise as ship opens fire on small boat said to have ignored warnings

One man was killed and three others were injured after a US Navy ship in the Persian Gulf opened fire as it was approached by what appeared to have been a fishing boat last night.

The ship is part of the Fifth Fleet, which has increased its patrols in the Gulf region because of heightened tensions with Iran. Both the US and Iran have warned each other against provocative naval activity in the Straits of Hormuz, which is a crucial supply line for oil shipping at the entrance to the Gulf.

The US Navy said a small motorised boat ignored several warnings to halt its advance in waters some 30 miles south-west of Dubai before a security team aboard the Fleet's supply vessel Rappahannock fired rounds from a .50-caliber machine gun.

The official Navy response did not speculate on the reasons for what it said had been the small vessel's disregard of its warnings – including warning shots – or whether its crew could have mistaken the vessel for an Iranian military one.

Associated Press reported from Dubai that the small vessel, a white-hulled civilian craft about 30-feet long had docked after the incident in a Dubai fishing port and that dozens of police officers and Emirates officials had crowded round it. Iran's Revolutionary Guard is known to use small but fast-moving vessels in the Gulf.

A spokesman for the Fifth Fleet told the New York Times last night that the incident was being investigated and added: "In accordance with Navy force protection procedures, the sailors on the United States Navy supply ship used a series of non-lethal, pre-planned responses to warn the vessel before resorting to lethal force."

On Sunday, Iran repeated its threats to close the Straits of Hormuz, through which 40 per cent of maritime oil exports pass, unless the sanctions imposed as a result of Iran's nuclear programme are revoked.

The US military has stepped up its presence in the region with additional fighter jets, deploying a floating naval base as well as doubling the number of minesweepers in recent months. Before yesterday's incident the Pentagon announced it was speeding up – by about four months – the deployment of an aircraft carrier strike group to the region, while insisting that it was not doing so solely because of tensions with Iran.

"This is about a wide-range of US security interests in the region," Pentagon spokesman George Little said.