"Haftar's supporters must understand that they have lost their gamble" by backing the eastern military strongman, said Fayez al-Sarraj, who heads the Tripoli-based Government of National Accord (GNA).
Foreign governments supporting Haftar will only "prolong the war and create deeply rooted hatred that will be difficult to overcome", Sarraj added.
States including the United Arab Emirates, Egypt and Jordan have bolstered Haftar, while the GNA is backed by Turkey and Qatar.
Russia is alleged to have sent several thousand mercenaries to support Haftar, claims denied by Moscow.
In a news conference broadcast live on television, Sarraj also charged that Haftar "was not a partner for peace".
His comments came days after the UN Security Council passed a resolution calling for a "lasting ceasefire" in Libya, where in April Haftar launched an offensive to seize Tripoli from the GNA.
Sarraj welcomed Wednesday's vote but accused Haftar's forces of repeatedly violating a fragile truce observed since January 12.
The UN resolution also called for continued negotiations by a joint military commission set up in January between the two sides, with the goal of achieving a "permanent ceasefire".
"We cannot negotiate as long we are being bombarded by aircraft, and there is bloodshed and the destruction of infrastructure," he said.
Sarraj also called for "severe international measures" to put an end to the ceasefire violations, otherwise the GNA "will be forced to retaliate", he said.
A day after the UN vote, rival forces clashed in Tripoli causing civilian casualties.
Weapons have continued to flow into the country despite world leaders agreeing at a January summit in Berlin to end all foreign interference in Libya and uphold a UN arms embargo.
Libya has been subject to a much-abused arms embargo since 2011, when a NATO-backed uprising toppled longtime dictator Moamer Kadhafi.
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