Gunmen have shot dead a Libyan marine colonel in the eastern city of Benghazi where militants have increasingly targeted security forces in a challenge to central government control.

Libya has struggled to curb rival armed factions in Benghazi, where the 2011 revolt that overthrew Muammar Gaddafi began and where the U.S. ambassador was killed during an Islamist assault on a U.S. diplomatic mission a year ago.

Today's attack killed Col. Salih Al Hidary from the Libyan Marines force, and followed assaults that killed two other officers in Benghazi a few days earlier.

"He was driving his car with his son when he was targeted," said a senior Libyan security source. "Many bullets penetrated his body, he died immediately, while his son is in intensive care with a gunshot wound to the head."

On Sunday, an air force officer was killed when a bomb was attached to his vehicle in Benghazi, while a colonel working for the Libyan intelligence was gunned down in front of his house as he left for work in the city.

A police patrol was also targeted in an rocket-propelled grenade attack at one of the main entrances to the city, though no injuries were reported, security sources said.

To help maintain security, Libya's government relies on militias made up of thousands of Libyans who took up arms against Gaddafi. But these rival groups have often involved into security threats themselves.

An OPEC country, Libya is also struggling to put an end to protests and strikes by oil facility guards and armed activists that have crippled crude operations in the east. Production is now at 700,000 barrels per day, less than half the usual output.