The United States expressed concern about the Russian mercenaries' interference in Libya's energy facilities and oil fields, calling them "a direct assault on Libya’s sovereignty and prosperity."

U.S. State Department Spokeswoman Morgan Ortagus on Monday shared a tweet saying: "We share Libya's National Oil Corporation's deep concern about the interference of Russian Ministry of Defense's proxy Wagner and foreign mercenaries against Libyan facilities and personnel at the (El-Sharara) oil field. This is a direct assault on Libya’s sovereignty and prosperity."

She also shared another message that the U.S. embassy in Libya tweeted Friday. "As responsible actors in #Libya return to UN-facilitated ceasefire talks and demand an end to foreign interference, allowing the NOC to immediately resume operations is a critical step to reestablish Libyan sovereignty," the tweet said.

On Friday, the country's National Oil Corporation said Russian and other foreign mercenaries had entered the key El-Sharara oil field the previous day.

El-Sharara is under the control of forces loyal to Haftar, who is backed by Russia, Egypt and the United Arab Emirates.

Libya's National Oil Corporation (NOC) said on Monday there had been international talks to end a blockade on oil exports, imposed since January by eastern-based forces in the country's civil war, and that it was hopeful output would resume.

The negotiations involved the internationally recognized Government of National Accord (GNA), the U.N., the U.S. and regional countries, an NOC spokesman said in a statement said.