A 677-foot United States Navy refueling ship in the Persian Gulf opened fire on Monday with a .50-caliber machine gun on what appeared to be a 30-foot sport fishing boat after it ignored repeated warnings to stop, killing a crew member and causing a spike in oil prices that reflected the heightened tensions in the region between Iran and the United States.

The United States Fifth Fleet command in Bahrain said in a statement that the shooting took place near the coast of the United Arab Emirates, a close American ally. The statement did not specify whether there were casualties or identify the nationality of the smaller vessel. But an American defense official said one person was dead, and the official news agency of the United Arab Emirates identified the victim as an Indian fisherman. At least three other Indians aboard were wounded, the agency said.

The encounter, which the Fifth Fleet command said was under further investigation, came against a backdrop of increasingly belligerent warnings from Iran against a buildup of naval forces in the area by the United States, which considers the free flow of oil from the Persian Gulf a vital national interest. The tensions have grown this month because of new Western sanctions aimed at stifling Iran’s oil exports.

The larger vessel’s response also appeared to reflect heightened awareness of the potential threat of small powerboats piloted by suicide attackers, such as the one that blew a 40-foot hole in the side of the Navy destroyer Cole in October 2000 while it was refueling in the Yemeni port of Aden, killing 17 sailors.