"There is no zero except a zero delusion," Serraj said in a televised address. "There is no control or storming of Tripoli or its neighbourhoods," he added.
In April, Haftar ordered a military campaign to seize Tripoli. The onslaught has since largely descended into a stalemate.
The 76-year-old general has portrayed the offensive as part of an anti-terrorism campaign.
Haftar's opponents say he wants to establish a military dictatorship and crush dissent in Libya.
"It is high time to fold the page of the one-man rule and the one-track mindset," Serraj said Friday, without naming Haftar.
Pro-Haftar forces meanwhile claimed to have advanced in the southern section of Tripoli in the past hours.
An LNA-linked military media centre released footage showing what it said were Haftar's forces controlling military camps from rivals in south-eastern Tripoli.
For their part, Serraj's forces said they had foiled an attempt by Haftar loyalists to advance in southern Tripoli.
"Our troops have captured 13 [pro-Haftar] fighters, including African mercenaries," Nasser Amar, a Tripoli commander, told dpa.
At least 24 Haftar loyalists were also killed in the clashes, Amar added, without giving casualty figures for the Tripoli forces.
LNA forces have been engaged in fighting against Tripoli insurgents since early Friday in the southern suburb of Ain Zara, Libyan news portal Africa Gate reported.
Haftar’s helicopters were targeting pro-GNA militias there, the report said without giving details.
The latest escalation comes days after the UN envoy for Libya, Ghassan Salame, said Haftar's forces were getting closer to scoring a major victory in the Libyan conflict thanks to military help from Russia.
Libya slid into anarchy after a 2011 armed revolt that toppled long-time dictator Moamer Gaddafi.
The oil-rich country has at least two rival administrations: the UN-backed government based in Tripoli, and the other based in the eastern city of Tobruk, which is allied with Haftar.
Egypt, the United Arab Emirates and Saudi Arabia are seen as aligned with Russia in backing Haftar, whereas Qatar is believed to support the Tripoli government, alongside Turkey.
Last month, Serraj signed a controversial security and maritime cooperation pact with Turkey. On Friday, EU leaders, meeting in Brussels, condemned the deal, saying it breaches international laws.
Germany is preparing to host a conference on Libya in Berlin next year, which aims to facilitate a ceasefire by bringing together regional powers involved in the conflict.
On Friday, German Chancellor Angela Merkel discussed Libya with French President Emmanuel Macron and Italian Prime Minister Giuseppe Conte on the sidelines of the EU summit in Brussesl.
In a joint statement, the three leaders called on “all Libyan and international parties to refrain from taking military action, genuinely commit to a comprehensive and lasting cessation of hostilities and re-engage into a credible UN-led negotiation."
They also called for the Berlin conference to take place "with no further delay."
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