NAIROBI (Reuters) - Somali pirates hijacked a U.S.-flagged, Danish-owned container ship on Wednesday with 20 American crew on board in a major escalation in attacks off the lawless Horn of Africa nation, officials said.

The 17,000 tonne Maersk Alabama, a U.S.-flagged, Danish-owned container ship in an undated image, taken when the ship was named differently, released to Reuters on April 8, 2009. REUTERS/Maersk Line Ltd/Handout

The 17,000 ton Maersk Alabama was seized off Mogadishu far out in the Indian Ocean, but all its crew were believed to be unharmed, Andrew Mwangura, coordinator of the Mombasa-based East African Seafarers’ Assistance Program, told Reuters .

Denmark’s A.P. Moller-Maersk confirmed the U.S.-flagged Maersk Alabama had been attacked by pirates about 500 km (300 miles) off Somalia and was presumed hijacked. The company said it had 20 American crew on board.

In Washington, White House staff were closely monitoring the situation and assessing what steps to take.

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

Among the ship’s cargo were 400 containers of food aid, including 232 containers belonging to the U.N.’s World Food Program that were destined for Somalia and Uganda.

In the latest wave of pirate attacks, gunmen from Somalia seized a British-owned ship on Monday after hijacking another three vessels over the weekend.

In the first three months of 2009 just eight ships were hijacked in the strategic Gulf of Aden, which links Asia, the Indian Ocean and the Red Sea to Europe via the Suez Canal.

Last year, heavily armed Somali pirates hijacked dozens of vessels, took hundreds of sailors hostage -- often for weeks -- and extracted millions of dollars in ransoms.

MORE ATTACKS

Foreign navies rushed warships to the area in response and reduced the number of successful attacks. But there are still near-daily attempts and the pirates have also started hunting further afield near the Seychelles.

On Monday, they hijacked a British-owned, Italian-operated ship with 16 Bulgarian crew on board.

Over the weekend, they also seized a French yacht, a Yemeni tug and a 20,000-tonne German container vessel. Interfax news agency said the Hansa Stavanger had a German captain, three Russians, two Ukrainians and 14 Filipinos on board.

The Maersk Alabama is owned and operated by Maersk Line Ltd, a Norfolk, Virginia-based subsidiary of A.P. Moller-Maersk and the world’s biggest container shipper.

A Moller-Maersk spokesman said it had been transporting general goods to Mombasa from Djibouti when it was attacked.

The pirates typically launch speed boats from “mother ships,” meaning they can sometimes evade warships patrolling the strategic shipping lanes and strike far out to sea.

They then take captured vessels to remote coastal village bases in Somalia, where they have usually treated their hostages well in anticipation of a sizeable ransom payment.

Pirates stunned the shipping industry last year when they seized a Saudi supertanker loaded with $100 million worth of crude oil. The Sirius Star and its 25 crew members were freed in January after $3 million was parachuted onto its deck.

Last September, they seized a Ukrainian cargo ship carrying 33 Soviet-era T-72 tanks and other heavy weapons. It was released in February, reportedly for a $3.2 million ransom.

Many of the pirates are based in northern Somalia’s semi-autonomous Puntland region, where the authorities called on Wednesday for more funds to tackle the gangs onshore.

“It’s better for the international community to give us $1 million to clear out the pirates on the ground, instead of paying millions of dollars to keep the warships at sea,” Puntland’s security minister, Abdullahi Said Samatar, told Reuters.