Doctors Without Borders (MSF) on Thursday said it was evacuating its staff from six hospitals in northern Yemen after 19 people were killed in an air strike on one of its facilities earlier this week.

Monday's Saudi-led coalition strike on Abs hospital in the rebel-held province of Hajja was the fourth and deadliest attack yet on an MSF facility in war-torn Yemen, according to the charity.

The decision to pull staff out "is never taken lightly", the Paris-based aid agency said in a statement, accusing the coalition of "indiscriminate bombings and unreliable reassurances".

"Given the intensity of the current offensive and our loss of confidence in the SLC's (Saudi-led coalition's) ability to prevent such fatal attacks, MSF considers the hospitals in Saada and Hajjah governorates unsafe for both patients and staff," it added.

The hospitals will continue to be manned by local workers and volunteers, MSF said.

Yemen has been gripped by unrest since Iran-backed Shiite Huthi rebels and allied loyalists of former president Ali Abdullah Saleh overran the capital Sanaa in September 2014.

The violence increased after a Saudi-led Arab coalition launched a military campaign in March last year to help shore up the government of President Abedrabbo Mansour Hadi.

The coalition stepped up its air strikes this month after UN-mediated peace talks between the rebels and Yemen's internationally backed government were suspended.

- 'Indiscriminate attacks' -

Monday's bombing of Abs hospital drew international condemnation, prompting the coalition to announce an independent investigation into the attack.

MSF said it had shared the hospital's GPS coordinates with all parties involved in the conflict.

"Coalition officials repeatedly state that they honour international humanitarian law, yet this attack shows a failure to control the use of force and to avoid attacks on hospitals full of patients," it said.

"MSF is neither satisfied nor reassured by the SLC's statement that this attack was a mistake."

It also accused all sides in Yemen's war of "indiscriminate attacks without any respect for civilians".

One MSF worker was among those killed in the Abs hospital attack, while another 24 people were wounded.

The group's emergency coordinator Laurent Sury told AFP that "several dozen" international and Yemeni MSF workers were affected by the decision to pull out of the six hospitals.

"Our aim is to open programmes, not close them, especially considering the enormous needs in the north," he said. "But today, the minimum security conditions can no longer be guaranteed."

He said that civilians were paying a heavy price in the conflict.

"Today in Yemen, you risk your life when you seek out care, whether you are a pregnant woman needing a Cesarean or child requiring antibiotics."

The hospital strike was the latest in a series of coalition raids that have allegedly hit civilian facilities -- including a school in the rebel stronghold of Saada on Saturday where 10 children were killed.

The UN says more than 6,500 people, mostly civilians, have been killed since last March and more than 80 percent of the population needs humanitarian aid.