The evacuation of 24 critically ill Yemenis from the rebel-held capital was delayed Friday, the World Heath Organization said, citing technical reasons. "It is unfortunate that the flight did not take off today as planned," a WHO spokesperson told AFP. "We are making every effort and working very hard to ensure the flight does take off as soon as possible."

The spokesperson said the evacuation would "most likely happen in the coming day or days". "We urge everyone engaged in this effort to allow this medical air bridge operation to happen," the WHO spokesperson said. No further details on the reasons for the delay were provided.

A first "mercy flight" evacuated seven children from Sanaa on Monday for medical treatment in the Jordanian capital Amman.

The United Nations is eager to build the necessary confidence between the warring parties to enable more such evacuations. A WHO source confirmed earlier on Friday that a plane carrying 24 patients and their companions was scheduled to leave later in the day from Sanaa to Amman.

A Saudi-led coalition that has been fighting in support of the beleaguered Yemeni government has kept Sanaa airport closed to commercial flights since 2016. But in November, the coalition announced that it was prepared to allow medical evacuations from the airport as a confidence-building measure to support UN peace efforts.

"This is the first of what we hope will be a number of flights in the medical air bridge," UN Resident Coordinator for Yemen Lise Grande told AFP after Monday's flight. She said she hoped there would be more evacuations in the following days, to Egypt as well as Jordan.

The reopening of Sanaa airport is a key demand of the Huthi rebels and one of the issues being pursued by UN mediators as they seek to relaunch peace talks.