The top commander of the Libyan army's special forces said on Monday his troops had joined forces with renegade general Khalifa Haftar, who has said he wants to purge the North African country of militant Islamists.
The announcement gives a boost to a campaign by Haftar, who has been denounced by the Tripoli government as attempting to stage a coup in the oil producer.
It had been unclear how many troops supported Haftar, whose forces launched an attack on Islamist militants in Benghazi on Friday in which more than 70 people died.
Militiamen apparently allied to Haftar also stormed parliament in Tripoli on Sunday and fought for hours with rival militiamen.
"We are with Haftar," Wanis Bukhamada told Reuters in the eastern city of Benghazi. On live television he had earlier announced his forces would join "Operation Dignity", as Haftar calls his campaign.
The special forces are the best trained troops of Libya's nascent army. They have been deployed since last year in Benghazi to help stem a wave of car bombs and assassinations, but struggled to curb the activities of heavily-armed Islamist militias roaming around the city.
An air base in Tobruk in Libya's far east also said it had joined Haftar's force - a significant move since it remains unclear how much backing Haftar has within Libya's regular armed forces and the powerful brigades of former rebels who toppled Muammar Gaddafi in 2011.
A statement from the air base said its personnel wanted to fight "extremists", echoing Haftar's rhetoric.
"The Tobruk air force base will join ... the army under the command of General Khalifa Qassim Haftar," the statement said. Staff at the air base confirmed its authenticity.
Since the end of Gaddafi's one-man rule, the main rival militias of ex-rebels have become powerbrokers in Libya's political vacuum, carving out fiefdoms and flexing their military muscle.