A man holds a picture of General Khalifa Haftar during a demonstration in support of the Libyan army under the leadership of General Khalifa in Benghazi, Libya November 6, 2015. REUTERS/Esam Omran Al-Fetori

TUNIS (Reuters) - The United States and five European powers on Monday urged forces loyal to eastern Libyan commander Khalifa Haftar to withdraw from several key oil ports seized from a rival force over the weekend.

Fighters loyal to Haftar took control of the ports of Ras Lanuf, Es Sider, Zueitina and Brega, displacing a force allied to the United Nations-backed government in Tripoli. The seizure threatened to reignite conflict over Libya’s oil resources and disrupt efforts by the U.N.-backed Government of National Accord to revive oil production and exports.

“We call for all military forces that have moved into the oil crescent to withdraw immediately, without preconditions,” the United States, France, Germany, Italy, Spain and Britain said in a joint statement.

The western powers condemned the attacks on the ports and affirmed their intent to enforce a U.N. Security Council resolution designed to prevent what they called “illicit” oil exports.

Eastern Libyan factions allied to Haftar had previously tried to export oil independently of the government and the National Oil Corporation in Tripoli.

Libya’s oil infrastructure, production and export must remain under the control of the corporation acting under the Government of National Accord, the western countries said in their statement.

U.N. Libya envoy Martin Kobler, in a separate statement, expressed “grave concern” at fighting around the oil ports, and called on forces who seized the terminals to refrain from further military escalation.

“Attacks on the oil terminals further threaten the stability and lead to a greater division of the country,” Kobler said. “They further restrict the oil exports and add to people’s suffering.”