UN envoy Martin Kobler said he was prevented from travelling Wednesday to the Libyan capital for his work on the installation of a new unity government.
Tripoli is under the control of an unrecognised administration backed by a coalition of militias including Islamists that opposes the new government from starting work inside the country.
"UN #Libya envoy supposed to arrive today to discuss moving UN-backed unity government to #Tripoli," the envoy wrote on Twitter.
"Again had to cancel flight to Tripoli... UN must have the right to fly (to) Tripoli," he said, without specifying what had blocked the mission.
Signup to our newsletter and follow us on Facebook and Twitter!
In Tripoli, the head of Libya's unrecognised government, Khalifa Ghweil, said that the decision to "postpone" the envoy's visit had come from his administration.
The cabinet had approved Kobler's visit but it was postponed as there had been no response to requests for "protocol" to be fulfilled, he said, without elaborating.
Libya has had two rival administrations since mid-2014 when the recognised government was forced from Tripoli to the far east after the Fajr Libya militia coalition overran the capital.
The United Nations is pushing Libya's rival politicians to accept a unity government created under a power-sharing deal announced in December.
It has not been formally endorsed by either parliament but last Saturday the government announced it was taking office on the basis of a petition signed by Libya's elected lawmakers.
Libya has descended into chaos since the 2011 ouster of longtime dictator Moamer Kadhafi, allowing extremist organisations including the Islamic State group to gain ground.