The evacuation of 24 critically ill Yemenis from Sanaa, the capital held by the Iran-backed Houthi militias, was delayed Friday, the World Health 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 development confirms statements by informed sources to Asharq Al-Awsat on Wednesday that accused the Houthis of obstructing the scheduled mercy flights from Sanaa to Amman and Cairo.

The flights were launched through an initiative by the Saudi-led Arab coalition.