The UN envoy to Yemen called Wednesday for "serious engagement" to uphold the war-torn country's truce, which has offered a rare respite from violence.

Hans Grundberg spoke at the end of his first visit to the rebel-held capital, where he held talks with Huthi rebel leaders.

The two-month ceasefire took effect 11 days ago.

"While we see that the truce is broadly holding, we need to be mindful of the challenges, too," the Swedish diplomat warned at the end of his three-day visit.

"We are relying on the parties' continued commitment and serious engagement in upholding the truce."

The renewable ceasefire comes seven years after a Saudi-led coalition began its military intervention in Yemen, after the Iran-backed Huthis took control of swathes of the country including Sanaa in 2014.

Grundberg said oil tankers had begun arriving at the port of Hodeida, one of the terms of the truce intended to ease the "fuel crisis" in Sanaa and elsewhere.

"Intense work" is underway for Sanaa airport's first commercial flight in six years, another feature of the pause in fighting, Grundberg said, while talks have started on reopening key roads in Taiz and other governorates.

"Despite reported violations that we are concerned about, we have seen a significant overall reduction in hostilities and no confirmed reports of air strikes or cross-border attacks," Grundberg added.

In another hopeful sign, Yemen's president last week handed his powers to a new leadership council tasked with holding peace talks with the rebels.

More than 150,000 people are estimated to have been directly killed and millions displaced by the fighting, triggering what the United Nations calls the world's worst humanitarian crisis.