"The victory flag will soon be raised in our capital... the meeting with victory is so close to the goal for which so many lives have been sacrificed," he said in a speech broadcast by pro-Haftar Al-Hadath television.
Haftar, whose forces control Libya's east and much of the country's south, launched an offensive in April to take the capital.
His fighters have faced fierce resistance from forces backing the Tripoli-based Government of National Accord (GNA).
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At least 1,000 people have been killed in the battle for Tripoli, according to the World Health Organization.
Dressed in a white uniform adorned with medals, Haftar told his forces to respect Tripoli residents but to be "ruthless and without mercy" with those who have taken up arms against them.
Forces loyal to the GNA said they fought off a "major" attack Monday led by Haftar that left casualties on both sides.
Haftar's self-styled Libyan National Amy meanwhile said they had "made progress" on the front lines to the south of Tripoli.
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