Raisedon Zenenga, assistant secretary-general and mission coordinator of the U.N. Support Mission in Libya, called on participants to pursue the effort, describing the talks as "heated debate" marked by threats of walk outs, Reuters reported.
"The people of Libya will certainly feel let down as they still aspire to the opportunity to exercise their democratic rights in presidential and parliamentary elections on 24 December," Zenenga told the closing session.
The talks, held at a hotel about 15 kilometres (9 miles) from Geneva, had been extended into a fifth day on Friday with delegates struggling to agree. They had been expected to establish the constitutional basis for presidential and parliamentary elections by July 1.
But delegates and U.N. officials said they could not agree among themselves on several proposals circulating, prompting organisers to extend the talks originally planned to last four days.
The elections would be a critical step in international efforts to bring stability to Libya, which has been in turmoil since a 2011 uprising against Muammar Gaddafi.