Lawmakers based in areas of eastern Libya controlled by the Libyan National Army (LNA) said on Monday they would not participate for now in peace talks with politicians allied to the internationally recognised government.
The eastern politicians said the United Nations, which is brokering the talks in Geneva, had not approved all the members of their proposed delegation.
The lawmakers' refusal is the latest blow to U.N. efforts to end an almost year-long offensive by LNA led by commander Khalifa Haftar aimed at taking the capital Tripoli.
The United Nations had planned to bring together lawmakers from both sides of Libya's conflict on Wednesday to end the fighting over Tripoli as part of a dialogue encompassing military, political and economic strands.
Tripoli government meanwhile described Haftar as a "war criminal" and demanded that he withdraw his forces threatening the capital.
The government, which nearly walked out of the Geneva talks last week, blamed Haftar's forces for shelling Tripoli's port shortly after the negotiations got underway.
Senior military figures on both sides on Sunday agreed to submit a draft ceasefire agreement to their leaderships before meeting again next month, the U.N. Libya mission, UNSMIL, said in a statement.
The eastern lawmakers refused to take part in Wednesday's talks as UNSMIL had approved only eight of their 13 delegates, Ahmaida Erouhma, head of the Geneva committee at the eastern parliament, told reporters in the eastern city of Benghazi.
UNSMIL spokesman Jean El Alam said the talks would go ahead as scheduled but declined to comment specifically on the participation of either side.
"Many participants have already arrived in Geneva and we hope all invited participants follow suit," he said.
The two sides have so far been seated in separate rooms, with U.N. envoy Ghassan Salame shuttling between them.
The Tripoli government insisted that Haftar must pull back his troops from front lines around Tripoli, foreign affairs minister Mohamed Syala told journalists.
Prime Minister Fayez al-Serraj earlier called Haftar a "war criminal" at a meeting of the U.N. Human Rights Council in Geneva.
"Children have lost their right to education due to the shelling and the closure of schools because of the attacker and those who fund the attacker and provide weapons, these must be held accountable," Serraj said.
Nearly nine years since NATO-backed rebels overthrew Muammar Gaddafi, Libya is still without a central authority in control of its towns and cities. Armed groups patrol the streets, while rival governments are in place in Tripoli and the east.
Haftar is allied to a rival government and parliament in eastern Libya, which is home also to a parallel central bank and oil company.
Both sides are backed by foreign powers. Haftar's Libya National Army (LNA) faction has support from Egypt and the United Arab Emirates, while Turkey has sent soldiers and arms to help the Tripoli government.
* This story has been edited by Ahram Online