A heavily armed militia has taken over a runway at Libya's main airport in the capital, Tripoli, demanding the release of a leader who went missing on Sunday.
But as negotiations failed to progress, security forces and militiamen from other brigades entered the airport vowing to oust the group by force.
The al-Awfia brigade earlier occupied the airport runway with armoured vehicles, forcing flight diversions.
Gunfire was heard and correspondents say the situation is developing fast.
The BBC's Rana Jawad in Tripoli says it is unclear if gunmen are firing at one another or in the air.
Dozens of militiamen from the al-Awfia group have been occupying the airport for hours.
But our correspondent witnessed about 30 armoured vehicles carrying a mixture of security forces and militiamen from various armed brigades entering the airport.
The al-Awfia group is refusing to leave until its demand for a senior commander to be released is met.
The situation at the airport is volatile and developing quickly. As we were leaving, we saw about 30 armoured vehicles heading rapidly into the airport.
The trucks were carrying a mixture of national army personnel, members of militias such as the Tripoli Brigade and armed men from a group called the Supreme Security Council.
One member of the Tripoli Brigade said they were moving in to take out the militia by force as negotiations had failed.
There was a lot of gunfire and as we were moving away, one bullet flew right past us.
But it is unclear whether al-Awfia's commander has been kidnapped or is being held for questioning by the government.
The brigade has placed a pick-up truck mounted with an anti-aircraft gun underneath each of the six planes on the tarmac, our correspondent says.
"They are on the runway, in the car park, everywhere," an official told Reuters news agency.
A representative from the western town of Tarhouna, where the brigade is based, is at the airport to negotiate with the rebels and convince them to give up their action.
But there is no sign of progress in the negotiations.
Airport sources told the BBC that at least three airlines have cancelled their flights.
Several international airlines have resumed flights to Libya since the end of the conflict which toppled Muammar Gaddafi.
Libya's interim government seized control of the airport in Tripoli in April, taking over from militia fighters who had been providing security in the absence of an official force.
The incident comes as Libya's governing National Transitional Council prepares for to hold elections for a constituent assembly in the coming weeks.