|The tanker as it was taking in oil at the Libyan coast.|
The operation was approved by Barack Obama and was conducted just after 2am GMT on Monday in international waters south-east of Cyprus, Kirby added.
The tanker had a North Korean flag but Pyongyang has denied any responsibility for the tanker, which was carrying oil owned by the Libyan government's National Oil Company.
"The ship and its cargo were illicitly obtained from the Libyan port of As-Sidra," the Pentagon statement read. The tanker would soon be en route to a port in Libya with a team of US sailors on board.
The Morning Glory, which departed from the eastern Libyan port of Al-Sidra – controlled by rebels seeking autonomy from the authorities in Tripoli – is reported to have been carrying at least 234,000 barrels of crude oil.
The ship was operated by an Egypt-based company that was allowed to temporarily use the North Korean flag under a contract with Pyongyang, North Korean state news agency KCNA said.
Pyongyang had "cancelled and deleted" the ship's North Korean registry, as it violated its law "on the registry of ships and the contract that prohibited it from transporting contraband cargo".
Former Libyan prime minister Ali Zeidan fled to Europe after parliament voted him out of office last Tuesday over his failure to stop rebels exporting oil independently. The General National Congress (GNC), Libya's transitional assembly, agreed that Defence Minister Abdallah al-Thinni would be acting prime minister for two weeks.