Somali pirates say they have hijacked a yacht owned by a U.S. couple that sails around the world distributing bibles to remote villages.

Pirates hijacked the S/V Quest on Friday, and are said to be holding four U.S. citizens hostage. A Somali pirate who identified himself as Bile Hussein said Saturday pirates from the country's northern Puntland region captured the yacht, and were expected to make landfall in Somalia on Sunday "if no problems happen on their way."

According to a website chronicling the voyage of a yacht bearing the name S/V Quest, it is the floating home of Jean and Scott Adam, a retired American couple who have been sailing the globe since 2002. The site states the pair took on two new crew members last year.

The couple write on their website that they run a Bible ministry, and have been delivering Bibles to schools and churches in remote communities from Alaska to New Zealand.

In their itinerary for 2011, the couple planned to sail from Sri Lanka to the Mediterranean, via the Suez Canal. Their yacht was apparently en route from India to Oman when it was attacked.

"Djibouti is a big refueling stop," Jean Adam wrote describing plans to land in the tiny East African nation directly north of Somalia. "I have NO idea what will happen in these ports, but perhaps we'll do some local touring. Due north is the Red Sea where we plan to tuck in when winds turn to the north."

Matt Goshko, a spokesperson at the U.S. embassy in Nairobi, Kenya, which also deals with issues in Somalia, said U.S. officials understand that there are four U.S. citizens aboard the ship.

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

Early Saturday, the deputy commander of the U.S. Navy's 5th Fleet told CNN that the U.S. military is prepared to intervene to rescue the couple if they are in fact aboard the hijacked vessel.

On Friday, Interpol's maritime piracy task force announced a US$2.17 million commitment to help seven African nations fight piracy in the waters off the coast of East Africa.

But because Somali pirates typically attack cargo vessels plying the busy shipping lanes around the Horn of Africa -- commanding millions in ransom for their release -- most private yachts are left to navigate the area without the protection of international anti-piracy forces.

The Puntland region's security minister condemned the hijacking Saturday and appealed for assistance in dealing with the ongoing attacks.

"We are not capable of stopping piracy," said Gen. Yusuf Ahmed Khary. "They have expertise and can reach far beyond Somali coastlines. Puntland will do its best to track them down."

One of the most recent, and high-profile, cases of a Western couple being held for ransom involved Brits Paul and Rachel Chandler, who were released in November after spending 388 days in captivity. They were captured while sailing on their private yacht.

In addition to the Quest, Somali pirates are believed to currently be holding 30 ships and more than 660 hostages.

A recent study pegged the cost of piracy on the global economy at between US$7B and US$12B annually.

The Quest was captured two days after a New York court sentenced Somali pirate Abdiwali Abdiqadir Muse to 33 years in prison for his part in the 2009 hijacking of the Maersk Alabama.

That incident captured the world's attention when U.S. Navy sharpshooters killed two pirates holding the ship's captain, Richard Phillips. Muse was the only surviving pirate.

In the wake of Muse's sentencing, a pirate identified only as "Hassan" told the AP that other Americans should expect "regrettable consequences" in retaliation.

With files from The Associated Press