(CNN) -- Somali pirates hijacked a yacht with four Americans onboard in the Indian Ocean, U.S. military officials said Saturday.
The identities of the Americans were not immediately known, but the yacht, the S/V Quest, is owned by Jean and Scott Adam, according to Ecoterra International, a piracy watchdog group.
It is not clear whether the Adams -- who were on a worldwide cruise -- are onboard.
Authorities said the yacht was en route from India to Oman when it was captured Friday.
"If the owners are onboard, it would be a sad log for the couple on their seven-year world journey," Ecoterra International said.
The Adams website documents their worldwide voyage, which started in late 2004. It includes trips to New Zealand, China, Cambodia and Panama.
On the site, the Adams say their mission "is to allow the power of the word to transform lives ... (and) seek fertile ground for the word and homes for our Bibles".
A post earlier this year said they would refuel in Djibouti waters, another pirate flash point.
"Djibouti is a big refueling stop," said the post, which is not dated. "I have no idea what will happen in these ports, but perhaps we'll do some local touring".
U.S. military officials said they are keeping an eye on the situation.
"We are aware of the situation and we are monitoring the situation carefully," said Bob Prucha, a spokesman for the United States Central Command. "We have been aware for some time".
Piracy has flourished off the coast of Somalia, which has not had an effective government for two decades. In April 2009, pirates seized the U.S.-flagged Maersk Alabama, leading to a standoff in the Indian Ocean.
U.S. forces moved to rescue American Capt. Richard Phillips after seeing a pirate aiming a weapon on his back, officials said at the time. Three pirates were killed and one was arrested.
The Somali man arrested was sentenced to more than 30 years in prison. CNN
