Now they've done it: US Military now tracking pirates.

When Somali pirates skimmed across the Indian Ocean and muscled their way aboard a US food aid ship today, it had all the hallmarks of a depressingly familiar scene: a hostage cargo ship, a vulnerable crew and a well-organised team of brigands with the firepower and knowhow to seize a ship and demand a fat ransom.

But as a dramatic tussle on the high seas played out tonight, it became clear this was a startlingly different confrontation to the regular string of hijackings and hostage-takings that have plagued the waters off the Horn of Africa in recent months.

The standoff was apparently defused when the 20-man crew turned on their captors and managed to overpower them, seizing one pirate and sending three others fleeing for their dinghy.

But the episode remained unresolved tonight: members of the crew said the pirates had escaped with the captain in tow, and the crew was negotiating for his return, offering food or anything else. The crew had held a pirate prisoner for 12 hours and released him in return for the captain, but the pirates had not kept their side of the bargain.

A US warship, the USS destroyer Bainbridge, was on its way to the area this evening. "We are trying to hold them off until the US ship arrives," said Ken Quinn, second mate on the ship.

The drama began when the Maersk Alabama, a container vessel owned by the Danish shipping giant, was hijacked 280 miles southeast of Eyl off Somalia's eastern coast this morning. It was the pirates' sixth successful strike in the past fortnight. It was also the first US ship, and crew, to be seized by Somali pirates. Although hostages are seldom hurt while ransoms are negotiated, the kidnapping of Americans would pose serious concern in the White House.

Obama and the White House team had only arrived back in Washington at about 3am (EST) after a week-long tour of Europe and Iraq, but they monitored the crisis, facing the prospect of paying millions in ransom money, as other countries have done, or ordering military action.

Captain Joe Murphy, father of the ship's second-in-command, Shane, and a lecturer at the Massacusetts Maritime Academy, said today that his son and other crew members had turned the tables on the captors. His son had recently talked to a class at the academy about the dangers of piracy.

The World Food Programme said the ship's cargo included food aid due to be unloaded in Mombasa, Kenya.