The pirates used small boats to attack the vessel for several hours
Somali pirates have hijacked a ship with at least 20 US crew on board, after what maritime officials described as a sustained night-time attack.
The US-operated container vessel, Maersk Alabama, was seized in the Indian Ocean about 400 miles (645km) east of the capital, Mogadishu.
The Danish-owned ship's crew members are believed to be safe, the European Union's maritime security force said.
It is the sixth seizure in recent days, including a British and Taiwanese ship.
Among the cargo of the 17,000-tonne Maersk Alabama was UN food aid destined for Somalia and Uganda.
In Washington, the White House said it was closely monitoring the situation and assessing what steps to take.
Pirates typically hold the ships and crews until large ransoms are paid by the shipping companies - last year the firms handed over about £80m (£54m).
The huge increase in frequency of attacks has forced several navies to deploy warships in the Gulf of Aden.
But the BBC's Adam Mynott says the pirates have responded in the past few weeks by switching the focus of their attacks further out into the Indian Ocean.
In a statement, the AP Moller-Maersk Group confirmed its ship had been "attacked by pirates and presumed hijacked" at around 0600 BST on Wednesday.
"The vessel is deployed in Maersk Line's East Africa service network and was en route to Mombasa, when it was attacked approximately 500km off the Somalia coast," the statement said.
"Our initial concern is to ensure proper support of the crew and assistance to their families."
The ship was attacked by several small boats, with the incident apparently lasting for about five hours.
Maritime officials say the vessel took all possible evasive action before it reported that the pirates had boarded.
It is believed to be the first ship with Americans on board to be hijacked in recent times.
The BBC's correspondent in the region, Peter Greste, says hostages from previous pirate attacks have been largely well-treated, but the capture of Americans may change the equation.
Somalia is largely under the control of a group with links to al-Qaeda, our correspondent says, and they may choose to make a political point out of the situation.
A US Navy official told the Associated Press that the government is in contact with the shipping firm, finding out all the details of the attack.
Earlier this week pirates boarded a British-owned ship, the Malaspina Castle, in an area heavily patrolled by a European Union taskforce.
The taskforce, consisting of six ships and two planes, is among almost a dozen anti-piracy patrols in the area.
But pirates managed to seize 15 ships last month - a large increase on the two taken in January and February.
One expert believes the weather had a bigger impact than the patrols on the pirates.
Rashid Abdi, of the International Crisis Group (ICG), a Brussels-based think tank, said: "It was all down to the fact that the weather did not favour them.
"And now the weather has improved, they're back in business again."
More than 130 pirates attacks, including close to 50 successful hijackings, were reported in 2008, threatening one of the world's busiest shipping lanes.