The Libyan government has declared a "military zone" and deployed troops in the west of the country, after days of clashes between rival militias.
The authorities have called for an end to the violence, which has left at least 16 people dead since Monday.
The area includes the mountainous towns of Zintan, Mizdah and Shegayga, some 150 kilometres south of Tripoli.
It is the latest outbreak of fighting to test the new authorities since the toppling of Muammar Gaddafi last year.
The violence comes just weeks before the country is due to hold elections for a national assembly, the first free poll in decades.
In a statement, officials said the army had been authorised to use all necessary force to protect civilians in the mountainous area. They also called for an immediate ceasefire by all parties and the provision of humanitarian aid.
"The army is going there now to impose a ceasefire and protect civilians," the army's Col Hamed Zwei told Reuters news agency.
The BBC's Rana Jawad in Tripoli says the fighting is mainly between fighters from Zintan, backed by another tribe known as the Guntrara from the city of Mizdah, and armed members of the Mashashya tribe based in the town of Shegayga.
Tensions date from the Gaddafi-era, when one tribe was given land expropriated from another, she says.
The latest clash was triggered by the death of a man from Zintan after he was stopped at a checkpoint, which Zintan militias have blamed on the Mashashya tribe.
Sources said that incident took place after tanks were transported from a weapons depot in Mizdah, which lies close to Shegayga, to Zintan.
The number of fatalities is officially 14 but other sources have claimed to the BBC that dozens have been killed in recent days, our correspondent adds. At least 89 others have been wounded.
Residents from the Mashashya group have claimed that they are being shelled by Zintan forces, our correspondent says.
The ruling National Transitional Council (NTC), which took power last year, has been struggling to maintain security and stability in a country awash with weapons left over from the conflict last year.
Observers say many animosities between villages of tribes, which were repressed under Gaddafi's autocratic system, have re-surfaced since the end of his rule.