The U.N. support mission in Libya didn't name any specific nations, but said they included "several who participated in the Berlin Conference." Saturday's statement said these countries were supplying advanced weapons, armored vehicles and foreign fighters.
Libya sits on Africa's Mediterranean coast, and is divided between rival governments, each supported by various armed militias and foreign backers. It has the ninth largest known oil reserves in the world and the biggest oil reserves in Africa.
The weak but U.N.-recognized government in the capital Tripoli is backed by Turkey, and to a lesser degree Qatar and Italy. Rival forces loyal to military commander Khalifa Hifter receive support from the United Arab Emirates and Egypt, as well as France and Russia.
"Over the last ten days, numerous cargo and other flights have been observed landing at Libyan airports in the western and eastern parts of the country providing the parties with advanced weapons, armored vehicles, advisers and fighters," the U.N. statement said.
The U.N. warned that continuing to funnel arms into the conflict threatens the "fragile truce" in Tripoli. Hifter's forces have laid siege to the capital since last April. A cease-fire was brokered earlier this month by Russia and Turkey.
At the Berlin summit, many world powers with an interest in Libya pledged to halt foreign interference and honor the U.N. arms embargo.
Among those who attended the Berlin conference were Russian President Vladimir Putin, Turkish President Recep Tayyip Erdogan, French President Emmanuel Macron, Egyptian President Abdel-Fattah el-Sissi Italian Premier Giuseppe Conte, and U.S. Secretary of State Mike Pompeo.
The peace push followed a surge in Hifter's offensive against Tripoli, which threatened to plunge Libya into chaos rivaling the 2011 conflict that ousted and killed longtime dictator Moammar Gadhafi.
Earlier this month, powerful tribal groups loyal to Hifter also seized several large oil export terminals along the eastern coast as well as southern oil fields. The closure of Libya's major oil fields and production facilities has resulted in losses of more than $255 million in the six-day period ending Jan. 23, the country's national oil company said Saturday.