Ship hired by U.S. military fires warning shots in Gulf

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."

(Additional reporting by Andrew Gray in Washington and Mohammed Abbas in Manama, Editing by Chris Wilson)