NAIROBI, Kenya (AP) — Somali pirates on Wednesday hijacked a U.S.-flagged cargo ship with 20 American crew members onboard, hundreds of miles from the nearest American military vessel in some of the most dangerous waters in the world.

The crew was believed to be safe, according to the UK Maritime Security Centre (Horn of Africa), an organization run by the European Union’s Naval Force.

British maritime officials have been able to contact the 17,000-ton Maersk Alabama, according to a U.S. defense official in Washington who spoke on condition of anonymity because he was not authorized to speak on the record.

The ship was carrying emergency relief to Mombasa, Kenya, when it was hijacked, said Peter Beck-Bang, spokesman for the Copenhagen-based container shipping group A.P. Moller-Maersk. It was the sixth vessel seized within a week, a rise that analysts attribute to a new strategy by Somali pirates who are operating far from the warships patrolling the Gulf of Aden.

The company confirmed that the U.S.-flagged vessel had 20 U.S. nationals onboard.

Cmdr. Jane Campbell, a spokeswoman for the U.S. Navy’s Bahrain-based 5th Fleet, said that it was the first pirate attack “involving U.S. nationals and a U.S.-flagged vessel in recent memory.” She did not give an exact timeframe.

Press secretary Robert Gibbs said the White House was monitoring the incident closely and “assessing a course of action.”

“Our top priority is the personal safety of the crew members on board,” Gibbs said.

Pentagon spokesman Bryan Whitman said there had not yet been any communications from the pirates for ransom. But he would not go into military plans.

“I’m not going to speculate on any future military actions,” Whitman said, when asked what the U.S. military may do.

Whitman said there are still no U.S. Navy ships within view of the vessel, and instead they are still “hundreds of miles away.”

It was not clear whether the pirates knew they were hijacking a ship with American crew.

“It’s a very significant foreign policy challenge for the Obama administration,” said Graeme Gibbon Brooks, managing director of the British company Dryad Maritime Intelligence Service Ltd. “Their citizens are in the hands of criminals and people are waiting to see what happens.”

Capt. Joseph Murphy, a professor at the Massachusetts Maritime Academy, told the Cape Cod Times on Wednesday that his son, Capt. Shane Murphy, is second in command on the ship. Just a few weeks ago, Shane Murphy, a 2001 graduate of Mass Maritime, talked to one of his father’s classes about the dangers of pirates.

“He knows the potential danger and he talked with my students about that,” the elder Murphy told the newspaper. “He connected right away with the students.”

Joseph Murphy did not immediately return calls left by The Associated Press at his office.

The U.S. Navy said that the ship was hijacked early Wednesday about 280 miles (450 kilometers) southeast of Eyl, a town in the northern Puntland region of Somalia.

Noel Choong, who heads the International Maritime Bureau, a piracy watchdog group in Kuala Lumpur, said depending on the speed of the ship, and where the pirates wanted to take it, it could take a day or two to reach shore.

U.S. Navy spokesman Lt. Nathan Christensen said the closest U.S. ship at the time of the hijacking was 345 miles (555 kilometers)away.

“The area, the ship was taken in, is not where the focus of our ships has been,” Christensen told The Associated Press by phone from the 5th Fleet’s Mideast headquarters in Bahrain. “The area we’re patrolling is more than a million miles in size. Our ships cannot be everywhere at every time.”

A NATO official said from Brussels that the alliance’s five warships were patrolling the Gulf of Aden at the time of attack.

“That’s where most of the shipping goes through and we can provide most of the protection in that vital trade route,” said the official who asked not to be identified under standing rules.

The NATO official said the taking of the crude-filled Saudi supertanker Sirius Star also happened in open water far off the Somali coastline. The Sirius Star was released in January,

Somali pirates are trained fighters who frequently dress in military fatigues and use speedboats equipped with satellite phones and GPS equipment. They are typically armed with automatic weapons, anti-tank rocket launchers and various types of grenades. Far out to sea, their speedboats operate from larger mother ships.

Most hijackings end with million-dollar payouts. Piracy is considered the biggest moneymaker in Somalia, a country that has had no stable government for decades. Roger Middleton, a piracy expert at the London-based think-tank Chatham House, said pirates took up to $80 million in ransoms last year.

NATO has five warships that patrol the region alongside three frigates from the European Union. The U.S. Navy normally keeps between five to 10 ships on station off the Somali coast. The navies of India, China, Japan, Russia and other nations also cooperate in the international patrols.

NATO sees piracy as a long-term problem and is planning to deploy a permanent flotilla to the region this summer. On March 29, a NATO supply ship itself came under attack by Somali pirates who appear to have mistaken it for a merchant ship. The crew quickly overcame the attackers, boarded their boat and captured seven.

This is the second time that Somali pirates have seized a ship belonging to the privately held shipping group A.P. Moller-Maersk. In February 2008, the towing vessel Svitzer Korsakov from the A.P. Moller-Maersk company Svitzer was briefly seized by pirates.

Before this latest hijacking, Somali pirates were holding 14 vessels and about 200 crew members, according to the International Maritime Bureau.

The Combined Maritime Forces issued an advisory Wednesday highlighting several recent attacks that occurred hundreds of miles off the Somali coast and stating that merchant mariners should be increasingly vigilant when operating in those waters.

“While the majority of attacks during 2008 and early 2009 took place in the Gulf of Aden, these recent attacks off the eastern coast of Somalia are not unprecedented,” the advisory provided by Navy officials in Washington said. “An attack on the large crude tanker Sirius Star in November 2008 occurred more than 450 nautical miles off the southeast coast of Somalia.”

The advisory said the “scope and magnitude of problem cannot be understated.”

Douglas J. Mavrinac, the head of maritime research at investment firm Jefferies & Co., noted that it is very unusual for an international ship to be U.S.-flagged and carry a U.S. crew. Although about 95 percent of international ships carry foriegn flags because of the lower cost and other factors, he said, ships that are operated by or for the U.S. government — such a food aid ships like Maersk Alabama — have to carry U.S. flags, and therefore, employ a crew of U.S. citizens.