Fierce clashes in Libya killed 24 people on Friday after a paramilitary force led by a former general attacked Islamist militias in Benghazi.
Local media reported that government troops had also joined the attack against the Islamists.
But acting Prime Minister Abdullah al-Thinni condemned the operation as "a coup against the revolution".
Libya's leaders have struggled to bring stability to the country since Muammar Gaddafi was removed from power in 2011.
Benghazi, an eastern city that was at the heart of the anti-Gaddafi uprising, has been the scene of frequent clashes between the army and Islamist militias.
The force that carried out Friday's attack is under the command of former Libyan army colonel Khalifa Haftar.
At least 24 people were reported to have died in the clashes and hospitals officials said more than 120 people had been wounded.
Eyewitnesses told the BBC that an Islamist militia base was attacked by helicopters and warplanes.
But speaking at a news conference in Tripoli, acting Prime Minister Abdullah al-Thinni condemned the operation as illegal and an attempted coup.
"We have given orders to intercept any force trying to enter Benghazi because they don't have legitimacy from the state," Mr Thinni said.
"Air force units that bombed targets in Benghazi today did so illegally, without any orders from us."
'Flushing out terrorists'
Mr Haftar, who defected from Col Gaddafi's army and took charge of rebel forces during the 2011 uprising, now heads a group called the "National Army".
A spokesman said the group had launched "a large-scale operation to flush terrorists out of Benghazi."
Mr Haftar stirred rumours of a coup in February after appearing in military uniform to call for a presidential committee to be formed to govern until new elections.
The Tripoli government said he had no authority and threatened legal action against him.
The BBC's Rana Jawad in Tripoli says an uneasy calm has now returned to Benghazi but residents are concerned that the fighting is a sign that the regular clashes are escalating.
Libya has a complex web of militias, and some are loosely aligned to the army.
But the armed forces are largely split by region and city, and correspondents say they rarely take orders from the central authorities.
In a further sign of instability, Algeria closed its embassy and consulate in Tripoli on Friday, saying its diplomats faced a "real and imminent threat".
On Tuesday, Jordan's ambassador to Libya was freed after being abducted by gunmen in Tripoli weeks earlier.