(Adds Italians kidnapped, security forces on maximum alert in Tripoli)
TRIPOLI, Jan 18 (Reuters) - Libya is sending troops to the country's restive south after gunmen stormed an air force base in the region's biggest town, Prime Minister Ali Zeidan said on Saturday, following days of skirmishes between rival tribesmen and militias.
Highlighting turmoil in the North African country, two Italian construction workers were kidnapped by unknown attackers in the east, while authorities put security forces in the capital Tripoli on maximum alert.
Western powers fear the OPEC producer will slide into instability as the government struggles to contain heavily-armed militias, tribesmen and Islamists who helped topped Muammar Gaddafi in 2011 but refuse to disarm.
Weak border controls and a small army lacking equipment have turned Libya into a weapons smuggling route for al Qaeda in sub-Saharan countries and also a transit corridor for Islamist fighters heading to Syria's war.
Zeidan said a small group of gunmen had entered the air force base outside Sabha, 770 km (480 miles) south of the capital Tripoli, but the government was in control of the town and its civilian airport.
"This confrontation (at the air base) is continuing but in a few hours it will be solved," the prime minister told a televised address, without elaborating.
Zeidan said he had sent his defence minister to Misrata to instruct troops based there to move to the south. The central coastal city is home to some of the most experienced soldiers and militias, battle-hardened from the 2011 uprising.
"The troops from Misrata have been commissioned by the government to conduct a national task ... to spread security and stability in the region," he said in the address.
Western powers worry about instability in the sparsely populated south bordering Niger, Chad, Sudan and Egypt. People traffickers also use the desert borders to smuggle refugees into Libya from where many try to reach Italy by boat.
In Tripoli, security forces were put on maximum alert after the clashes in Sabha, a security spokesmen said, without elaborating.
In the volatile east, a security source said the two Italian construction workers were kidnapped in Derna, east of Benghazi, where they had been staying in a cement factory.
"There was a group of Libyan construction workers waiting for them on the highway east of Derna to fix a hole in the road, but the Italians did not arrive," the source said. "We are trying to establish the identity of the kidnappers, to find out about their demands."
Derna is a stronghold of radical Islamists. Residents temporarily blocked a road outside the town to protest against the kidnapping, a local activist said.
Benghazi, the main city in the east, has been rocked by a wave of assassinations of army and police officers as well as car bombs. Most Western nationals left the city after the U.S. ambassador was killed during an Islamist assault on the U.S. consulate there in September 2012.
A mix of militias and tribesmen has seized the main oil export ports in the east to press for political autonomy, drying up oil revenues, Libya's lifeline. (Reporting by Ulf Laessing, Ghaith Shennib and Feras Bosalum; Editing; Editing by Sophie Hares and Alister Doyle)