The United Nations has reported "positive" signs for renewing a truce that has eased suffering in war-torn Yemen, though there was no concrete breakthrough early Thursday, when the initial deal was set to expire.

Aid agencies and Western governments have urged the Yemeni government and Huthi rebels to extend the truce, which has significantly reduced the intensity of fighting in a conflict the United Nations says has triggered the world's worst humanitarian crisis.

"We have received preliminary, positive indications from the parties at this point," Stephane Dujarric, spokesman for UN chief Antonio Guterres, told a press briefing Wednesday.

But he did not elaborate, saying only that "as soon as we have something more concrete, we will share with you".

The United States warned on Tuesday that truce talks were in "trouble", heightening fears of renewed violence.

Yemen has been gripped by conflict since the rebels took control of Sanaa in 2014, triggering a Saudi-led military intervention in support of the beleaguered government the following year.

Aid groups in Yemen have touted the benefits of the truce, which went into effect in April.

The Norwegian Refugee Council reported last month it had cut civilian casualties by half.

On Wednesday, a Yemeni aircraft left the rebel-held capital Sanaa for Cairo on the first commercial flight between the two cities since 2016.

It is the seventh such flight under the truce, with the previous six all heading to the Jordanian capital Amman.

Beyond the flights -- a lifeline to Yemenis needing medical care abroad -- the truce has allowed oil tankers to dock in the rebel-held port of Hodeida, potentially easing fuel shortages in Sanaa and elsewhere.

But a provision for the rebels to ease their siege of Yemen's third-largest city Taez has yet to be implemented, to the anger of both the government and residents, who have held repeated protests in recent weeks.

The head of Yemen's presidential leadership council, Rashad al-Alimi, discussed the implementation of the truce with Guterres by telephone on Tuesday.

He urged the UN chief to "redouble the pressure on the Huthi militia to abide by its commitments to the truce, including opening roads to Taez", the official Saba news agency reported.

More than four million people have been displaced by the war, and 19 million stand to go hungry this year, Dujarric said Wednesday.

That includes "more than 160,000 who will face famine-like conditions", he said.