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

A U.S. 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 U.S. Embassy in Nairobi, Kenya, which oversees Somalia, said preliminary reports indicate there are four U.S. citizens aboard the Quest.

"All relevant U.S. 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.

Multimillion dollar ransoms are fueling 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. Pirates currently hold 30 ships and more than 660 hostages, not counting the attack against the Quest.

After the Maersk Alabama was hijacked in April 2009, Navy sharpshooters on the fantail of the USS Bainbridge fired on pirates holding Phillips, the ship's captain, killing two of them. The only pirate to survive that U.S. rescue was Abdiwali Abdiqadir Muse, the pirate who was sentenced to 33 years in prison this week.

The best known case of Westerners being held hostage in Somalia was that of Paul and Rachel Chandler, a British couple held for 388 days. The two, who were captured while sailing in their private yacht, were released in November.

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

The Adams website chronicles the couple's travels over the last seven years, from El Salvador and Panama in 2005 to Fiji in 2007 and Singapore and Cambodia last year. They most recently sailed from Thailand to Sri Lanka and India. Their website said they were on their way to Oman when they were taken. Djibouti — the tiny East African country directly north of Somalia — had been next on their list. A satellite tracking system the couple uses showed them docked in Mumbai, India on Feb. 1.

"Djibouti is a big refueling stop. I have NO idea what will happen in these ports, but perhaps we'll do some local touring," the couple's website says.

The couple runs a Bible ministry, according to their website, and have been distributing Bibles to schools and churches in remote villages in areas including the Fiji Islands, Alaska, New Zealand, Central America and French Polynesia.

They are members of the Marina del Rey Yacht Club in Marina del Rey, California, according to the website.

The prison sentence given to Muse this week could have implications on the hijacking of the Quest and the four Americans. Pirates have turned increasingly violent in their attacks, and naval officials say pirates have begun systematically torturing hostages and using them as human shields.

Earlier this week a Somali pirate told an Associated Press reporter in Somalia that pirates would target Americans in retaliation for the sentencing. The pirate, who identified himself by the name Hassan, said Americans would suffer "regrettable consequences."

Pirates have recently tied hostages upside down and dragged them in the sea, locked them in freezers, beaten them and used plastic ties around their genitals, the commander of the European Union anti-piracy force, Maj. Gen. Buster Howes told AP this month.

*Associated Press reporter Abdi Guled in Mogadishu, Somalia, contributed to this report.