GENEVA, July 18 (Reuters) - The United Nations called on Tuesday for the Libyan National Army (LNA) which controls the eastern part of the country to investigate summary executions of prisoners.

Its human right's body and voiced concern at the fate of those still in their custody.

The LNA is pushing to expand its presence in central and southern Libya, where it has been vying for control with forces linked to the U.N.-backed government in Tripoli and other opponents.

LNA leader Khalifa Haftar has gained ground with Egyptian and Emirati support, and Western states say Haftar must be part of any solution to Libya's conflict

"We are deeply concerned that, after recent fighting in Benghazi, people taken prisoner by members of the Libyan National Army, which effectively controls eastern Libya, may be at imminent risk of torture and even summary execution," U.N. human rights spokeswoman Liz Throssell told a news briefing.

Reports have suggested the involvement of Special Forces, a unit aligned with the LNA, "in torturing detainees and summarily executing at least 10 captured men", she said.

The Libyan National Army announced last March that it would conduct investigations into alleged war crimes but has not shared any information, Throssell said.

"We urge the LNA to ensure there is a full, impartial investigation into these allegations," she said.

Throssell also called on the group to suspend Mahmoud al-Werfalli from his duties as a Special Forces field commander pending the conclusion of such an investigation.

A video circulating on social media in March allegedly showed al-Werfalli shooting dead three men who were kneeling and facing a wall with their hands tied behind their backs, Throssell said.

In June, two further videos appeared to show summary executions carried out by LNA fighters on his orders.

"One of these videos, which emerged on 9 June, shows four men kneeling with their hands tied behind their backs who are shot dead as al-Werfalli watches," she said.

"The latest video, which was posted on social media this month, seems to shows LNA fighters kicking and taunting prisoners, while al-Werfalli is apparently heard accusing two men who have their hands tied behind their backs of belonging to terrorist groups," she said.

The LNA has declined comment on the images.

(Reporting by Stephanie Nebehay in Geneva; additional reporting by Aidan Lewis in Tripoli; editing by Jeremy Gaunt) ((Stephanie.Nebehay@thomsonreuters.com; +41 58 306 2161; Reuters Messaging: stephanie.nebehay.thomsonreuters.com@reuters.net; twitter @StephNebehay))