By Ayman al-Warfalli

BARSIS, Libya (Reuters) - A suicide bomber detonated a truck packed with explosives at an army checkpoint outside the city of Benghazi on Sunday, killing 13 soldiers in the first such attack in Libya's deepening turmoil.

Car bombs and assassinations of army and police officers are common in Benghazi, where troops have clashed regularly with militants from the hardline Islamist group Ansar al-Sharia.

But a suicide bombing would mark a shift in tactics to fit a pattern common in other Islamist struggles in the Middle East, but not in Libya either during or since the uprising that brought down Muammar Gaddafi.

The attacker blew himself up at a road checkpoint near a small military base in Barsis, some 50 km (30 miles) outside Benghazi, killing 13 people and wounding three, state news agency Lana said.

"A Toyota truck approached the checkpoint and parked there. There was a young man driving, but when the army troops went to check it out, the vehicle exploded," said Aymen al-Abdlay, a Benghazi army officer.

A Reuters reporter at the scene saw four destroyed cars and blood splashed on the tarmac.

The government postponed celebrations for Libya's independence day on December 24 and declared three days of mourning for the victims of what it called a "cowardly terrorist act," Lana said.

Two of the wounded were in serious condition, the agency said. Two people were still missing.

No group claimed responsibility for the attack.

But Ansar al-Sharia last month fought with soldiers who drove Islamists from Benghazi, where growing violence has further increased concerns about a wider descent into disorder in oil-producing Libya.

The militants are determined to impose their ultraconservative vision of Islam.

All those killed in the attack on Sunday were soldiers, medical sources and security sources said.

Ansar al-Sharia in Libya, an affiliation of Islamist and ultraconservative Salafist groups, has been blamed for the attack on the U.S. consulate in Benghazi in September 2012 when the U.S. ambassador and three other Americans were killed.

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An American school teacher was shot and killed by gunmen earlier this month while he exercised in the city.

Most countries have closed their consulates in Benghazi and some foreign airlines have stopped flying there.

Western diplomats worry that the violence in Benghazi will spill over to the capital Tripoli, which last month saw the worst fighting in months between militias.

Much of Libya's oil wealth is located in the east where many demand autonomy from the Tripoli government. Protesters in the east have taken over key ports, blockading much of the North African country's oil exports for months.

The government of Prime Minister Ali Zeidan is struggling to control militias and tribesmen which helped topple Gaddafi in a NATO-backed uprising in 2011 but kept their guns and often resort to force to make political demands.

Oil exports, Libya's lifeline, have fallen to 110,000 barrels a day, a fraction of the more than 1 million bpd in July as armed militias, tribesmen and minorities have seized oilfields and ports to press for more rights.

(Additional reporting by Ghaith Shennib in Tripoli; Writing by Ulf Laessing; Editing by Patrick Markey, Matthew Tostevin and Sonya Hepinstall)