BENGHAZI (Reuters) - A group that has previously shut down Libya's largest oilfield is threatening to do so again as international powers gather in Berlin for a summit on Libya, potentially taking nearly all the country's oil output offline.

The group Fezzan Anger, named for Libya's southwestern Fezzan desert region, was preparing to shut down the El Sharara and El Feel fields to push economic and security demands, its leader Bashir al-Sheikh said.

"We are now preparing and gathering to close Sharara and El Feel. So far we have not closed them," Sheikh said.

The statement came just two days after ports and fields in the east were shut down. The National Oil Corporation (NOC) said the closure was ordered by eastern military commander Khalifa Haftar's Libyan National Army (LNA) and would lead to the loss of 800,000 barrels per day (bpd), more than half the country's total production.

El Sharara has production of abound 300,000 bpd and El Feel produces some 70,000 bpd. Both fields are controlled by groups aligned with the LNA.

Haftar's LNA has been waging an offensive since April to capture the capital, Tripoli. It is one of two main factions that international powers want to agree to a ceasefire in Berlin on Sunday.

The move to close oil fields and ports is seen as a pressure tactic ahead of the summit, though the LNA has attempted to portray it as the result of popular pressure.

Eastern groups aligned with the LNA have long tried to gain control over oil exports and revenues, which are channelled through the central bank and the internationally recognised government in Tripoli.

The NOC has warned that any shutdowns could have a lasting impact.

"The oil and gas sector is the lifeblood of the Libyan economy and the single source of income for the Libyan people ... They are not cards to be played to solve political matters," NOC Chairman Mustafa Sanalla said on Friday.