Libya's UN-backed prime minister has vowed to defend the capital Tripoli as forces loyal to a rival advance from the east.
In a televised address Fayez al-Serraj accused General Khalifa Haftar of launching a coup, saying his troops would be met with "strength and power".
The rebels are on the outskirts of the capital and say they have seized Tripoli's international airport.
Tripoli is the base of the UN-backed, internationally recognized government.
Rebel forces are advancing on Tripoli in a multi-pronged attack from the south and west of the city, although they have reportedly been slowed by pro-government fighters.
Gen Haftar's Libyan National Army (LNA) troops seized the south of Libya and its oil fields earlier this year.
Gen Haftar - who was appointed the chief of the LNA under an earlier UN-backed administration - ordered his forces to advance on Tripoli on Thursday, as UN Secretary General Antonio Guterres was in the city to discuss the ongoing crisis.
Gen Haftar spoke to Mr. Guterres in Benghazi on Friday and reportedly told him that his operation would not stop until his troops had defeated "terrorism".
Prime Minister al-Serraj said his government had "extended our hands towards peace" while Gen Haftar had declared a coup.
Forces have been ordered "to deal with the threat of those striving to destabilize and intimidate civilians", he said, adding those responsible will be brought to justice.