Libya says Prime Minister Ali Zeidan has been kidnapped by a group of men believed to be former rebels.
The government said in a statement Mr. Zeidan was taken early Thursday from a Tripoli hotel where he lives to an unknown location. The motive was also not known.
A group of former rebels known as the Operations Room of Libya's Revolutionaries claimed responsibility, saying they "arrested" the prime minister.
Mr. Zeidan's abduction comes amid anger over Saturday's U.S. special forces raid in Libya that nabbed senior al-Qaida operative Abu Anas al-Libi, who is being held aboard a Navy ship.
Libya called on the United States to explain the operation, which it termed a "kidnapping." Mr. Zeidan said Tuesday his country wants to keep good relations with the United States, but that Libyan citizens should be judged at home.
U.S. Secretary of State John Kerry defended the capture, calling it "legal and appropriate." He said the Libyan government's complaints are unfounded, and that Libi will go before a court of law.
U.S. State Department spokeswoman Jen Psaki, who is traveling with Kerry, said Thursday the U.S. is looking into the prime minister's abduction and is in close touch with Libyan officials.
Libya has struggled with instability and violence since the ouster of longtime dictator Moammar Gadhafi in late 2011.
Rebel groups that fought against Mr. Gadhafi's forces have given rise to militias and militant groups who control parts of the country.
Attacks have targeted foreign diplomatic missions in Libya, including the Russian embassy last week, the French embassy in April and a U.S. consulate last year.