The governments of France, Italy, Germany, Britain and the United States said in a statement that they "agree that there is no military solution to the Libyan crisis" and expressed dismay that calls for a ceasefire had not been respected.
Dozens of people have been killed in Benghazi in days of fighting between militant groups, including Ansar al-Sharia, and pro-government forces led by former General Khalifa Haftar, who began an offensive on Wednesday, Reuters reported.
The joint statement voiced concern over Haftar's offensive and said Libya's "fight against terrorist organizations can only be sustainably addressed by regular armed forces under the control of a central authority."
Libya has failed to build up state security forces and disarm former rebels who helped remove Muammar Gaddafi, who ruled the country for 42 years until his downfall in 2011.
The United States and its four European allies also condemned Ansar al-Sharia and said "Libya's hard fought freedom is at risk if Libyan and international terrorist groups are allowed to use Libya as a safe haven."
The statement threatened sanctions against individuals who "threaten the peace, stability or security of Libya or obstruct or undermine the political process."
Libya also is struggling with two competing governments vying for control after armed groups from the western city of Misrata seized the capital of Tripoli in August, forcing the government of Prime Minister Abdullah al-Thinni to withdraw to the east.