Nairobi: The United States government said yesterday it was assessing possible responses after Somali pirates hijacked a yacht with four Americans on board in the Arabian Sea off the coast of Somalia.

Pirates hijacked the yacht Quest on Friday, two days after a Somali pirate was sentenced to 33 years in prison by a New York court for the 2009 hijacking of the Maersk Alabama. That case ended in a spectacular rescue when Navy sharpshooters killed two pirates holding the ship's captain, Richard Phillips.

The Quest is the home of Jean and Scott Adam, a couple who has been sailing around the world since December 2004, according to their website.

A US military spokesman at Central Command in Florida said: "We're aware of the situation and we continue to monitor it."

Matt Goshko, a spokesman at the US Embassy in Nairobi, which oversees Somalia, said preliminary reports indicate there are four US citizens aboard.

"All relevant US agencies are monitoring the situation, working to develop further information, assess options and possible responses," Goshko said.

Pirates have increased attacks off the coast of East Africa in recent years despite an international flotilla of warships dedicated to protecting vessels and stopping the pirate assaults.

Multi million dollar ransoms are fuelling the trade, and the prices for releasing a ship and hostages have risen sharply. One ransom last year was reported to be $9.5 million (Dh34.88 million). Pirates currently hold 30 ships and more than 660 hostages, not counting the Quest.

US officials will likely try to prevent the Adams' yacht from reaching Somalia, where their options to rescue the Americans become more limited.