TRIPOLI (Reuters) - Seven Egyptian Christians were found shot dead on a beach in eastern Libya after they were abducted from their apartments, security officials and local residents said, in the second such execution-style killing this year.

Three years after the revolution that ousted Muammar Gaddafi, Libya's weak government and army is struggling to impose state authority and control brigades of former rebels and Islamist militias in a country awash with weapons.

A police officer told Reuters the bodies were found with gunshots to the head outside Benghazi in the east, where assassinations, kidnappings and car bombs are common and Islamist gunmen are active.

"They were killed by head shots in execution style," a police officer said. "We don't know who killed them."

Local residents and an Egyptian worker, who asked not to be identified because of fears for their security, said unknown gunmen had arrived at the Benghazi building where the Egyptians lived and dragged them away after going door to door asking if residents were Christian or Muslim.

Security sources confirmed the Egyptians were Christians. No further details were immediately available about how they were killed or whether they were shot on the beach.

A spokesman for the Egyptian foreign ministry said the kidnappers had snatched eight Egyptians from their home but one had been able to escape.

It did not give the religion of those killed but published seven names which appeared to be Christian.

"Egypt expects from the Libyan side a speedy investigation into this gruesome crime ... to provide results as soon as possible and a prosecution of the culprits," the spokesman said.

No group claimed responsibility. Last month, a British man and a New Zealand woman were shot execution-style on another beach 100 km (60 miles) to the west of the capital Tripoli.

The hardline Islamist group Ansar al-Sharia is active in the east of Libya. Its Benghazi branch is listed as a foreign terrorist organization by Washington, which blames the group for the 2012 attack on the U.S. consulate in Benghazi.

(Reporting by Tripoli and Cairo newsrooms; Writing by Patrick Markey; Editing by Andrew Roche)