Navy Seals board rogue Libya oil tanker Morning Glory
The US has taken control of a tanker full of oil loaded from a rebel-held port in Libya, the Pentagon says.
The raid by Navy Seals took place in international waters south of Cyprus, said spokesman Rear Adm John Kirby.
The Morning Glory's evasion of a naval blockade at the eastern port of Sidra prompted Libya's parliament to sack Prime Minister Ali Zeidan last week.
In a separate incident on Monday, an attack on an army base in the city of Benghazi killed several soldiers.
A bomb went off as people were leaving a graduation ceremony for officers, police said.
If Washington was trying to prove that it only recognises Libya's elected authorities, it has succeeded in doing so with this Navy Seals operation.
The US has sent a clear message to both potential traders of illicit oil and to the armed groups blocking Libya's terminals that it will not be easy to sell the oil from rebel-held areas.
Libya matters to the US partly because a failed state would be viewed as yet another failed US adventure abroad, after it backed the uprising against Muammar Gaddafi.
The intervention will also help to dispel at least one of the many suspicions and rumours among Libyans - that the West will deal with anyone to get hold of crude oil.
It may also restore some of the Libyan government's credibility with people here, which has been lost over the past year. However, the dangers of this blockade escalating into an armed confrontation remain and it hinges on the government's next move.
The Morning Glory was the first vessel to have loaded oil from a rebel-held port since a separatist revolt against the central government in Tripoli erupted in July 2013. It is not clear where the tanker was headed.
Adm Kirby said the operation had been authorised by President Barack Obama and that no-one had been hurt.
"The Morning Glory is carrying a cargo of oil owned by the Libyan government National Oil Company. The ship and its cargo were illicitly obtained," he said, adding that it would now be returned to a Libyan port.
The vessel was flagged in North Korea but officials in Pyongyang said it had been deregistered because of the incident.
It was said to have been operated by an Egyptian company.
The BBC's Rana Jawad in Tripoli says the US move is likely to act as a deterrent to any further attempts to illicitly buy oil from the rebel-controlled ports.
She says that after backing the 2011 rebellion against Muammar Gaddafi, the US does not want Libya to become a failed state.
The Libyan government is still struggling to assert its authority on rebel groups that helped to overthrow Col Gaddafi.