Yemeni children in critical need of medical care were evacuated from the rebel-held capital Sanaa, in what the United Nations hopes will be the first of more mercy flights from the war-torn country.

The Secretary General of the Norwegian Refugee Council, Jan Egeland, described the flights as a "glimmer of hope", but said too many people had died waiting for medical evacuation.

The flights took two years of negotiations to set up, according to the UN humanitarian coordinator in Yemen, Lise Grande.

Speaking from Sanaa airport, which has been closed to civilian flights since 2015, she said: "There are thousands of patients who need this care. This is the first flight, there will be more.

"It's crucially important that this first flight has gone. All of us feel today that this is a major breakthrough and an indication of hope out of Yemen."

However, she added: "The real solution is to end the war."

15-year-old Abdallah Abed was one of 16 patients to be flown out on the first flight to Amman.

"I have kidney failure and I need a transplant," he said.

"God willing we travel today to Jordan for treatment."

Supervised by the United Nations and World Health Organization, flights from Sanaa will go to Amman in Jordan and Cairo in Egypt.

The WHO said the majority of the patients are women and children suffering from cancer and brain tumours, or who need organ transplants or reconstructive surgeries.

Yemen's internationally recognised government, backed by a Saudi-led military coalition, has been fighting Iran-backed Houthi rebels since 2014 when they seized control of the capital.

In November, the coalition, which controls Yemen's airspace, said patients needing medical care would be allowed to fly out of Sanaa.

The move was among confidence-building measures aimed at ending the five-year war that has killed thousands of people and displaced millions in what the UN has termed the world's worst humanitarian crisis.

The launch of the "air bridge" is a rare bright spot in Yemen, and a possible step towards reopening Sanaa airport, which is a key demand of the Houthis and one of the issues being pursued by UN-led mediators.

Abdullah Abed's father Abed Ali Murshid said: "Today is the start of the air bridge that we have been waiting for two years.

"Severe diseases in Yemen are many, and people need to travel.

"The United Nations must operate the air bridge regularly, send abroad patients suffering from diseases, open Sanaa airport, and end the blockade of Yemen."

The Houthis yesterday criticised the plan as inadequate, saying as many as 32,000 patients with serious illnesses were waiting to sign up for medical evacuations.

The WHO says three more flights have been scheduled this week, bound for Amman and Cairo.

The Norwegian Refugee Council welcomed the start of the humanitarian airlift, saying thousands of Yemenis had been handed a "death sentence" when the coalition closed down Sanaa airport.

"Today's move comes too late for thousands of Yemenis who died waiting to leave the country for urgent life-saving care," said Mohamed Abdi, the NRC's country director for Yemen.

"We hope that these medical flights will save the lives of other Yemenis. Many more are still waiting to get the healthcare they need.

"There is no justification for punishing very sick civilians by blocking them from accessing medical treatment.

"We hope that the recent increase in military escalations in northern Yemen does not put these flights under threat, and this small but significant step helps to build confidence between conflict parties.

"They need to end their political wrangling and fully reopen Sanaa airport for commercial flights."

The launch of the evacuations comes against a backdrop of a deterioration in Yemen's conflict, with fierce fighting around Sanaa that has erupted again, after a period of relative calm.

The UN Refugee Agency in Yemen estimates that more than 3,800 families have been displaced this year between 19 January and 2 February following a rapid escalation of hostilities.

Additional reporting AFP, Reuters