Pro-Haftar mercenaries were also deployed to the region and drones operated by the United Arab Emirates were flying over Tripoli.
Forces loyal to warlord Khalifa Haftar launched missile strikes to the south of Libya's capital, Tripoli late on Tuesday.
The Salah al Din and Ramla regions south of the capital were hit by missiles, Muhammed Kununu, military spokesman for the UN-backed Government of National Accord (GNA) said.
Pro-Haftar mercenaries were also deployed to the region and drones operated by the United Arab Emirates are flying over Tripoli, Kununu said.
He said government forces are strong enough to respond to Haftar's militia, and called on people who migrated from the region not to return to their homes during the clashes.
On January 12, the warring sides in the Libyan conflict announced a ceasefire brokered by Turkey and Russia.
All sides gathered Monday in Moscow to sign an agreement aimed at ending hostilities and starting political dialogue.
Following talks in Moscow, the head of the GNA, Fayez al Sarraj, signed a ceasefire deal late Monday, but Haftar left for Libya early Tuesday without signing.
Akile Saleh Issa, the speaker of the Haftar's rogue House of Representatives in the eastern city of Tobruk, said "the ceasefire in Libya is over and the war will resume."
Since the ousting of late ruler Muammar Gaddafi in 2011, two seats of power have emerged in Libya: one in eastern Libya supported mainly by Egypt and the UAE, and the other in Tripoli, which enjoys UN and international recognition.