The International medical charity Doctors without Borders (MSF) said eleven people were killed and 19 were injured when a hospital it supports in northern Yemen was hit by airstrikes on Monday.
"The blast immediately killed nine people, including an MSF staff member. Two patients died while being transferred to Al Jamhouri hospital," said an MSF statement.
It said that at the moment of the strike there were 23 patients in surgery, 25 patients in the maternity ward, 13 newborns and 12 children.
MSF confirmed that the hospital it supported in Abs, Hajjah province, was hit by an airstrike at 15:45 local time (1245 GMT).
It added that the hospital which it has been supporting since July 2015, was partially destroyed and all remaining patients and staff have been evacuated.
"The location of the hospital was well known, and the hospital’s GPS coordinates were repeatedly shared with all parties to the conflict, including the Saudi-led coalition," the statement said, adding that "Whether intentional or a result of a negligence, this is unacceptable.”
UN Secretary General Ban Ki-moon condemned the airstrike. Ban was "deeply disturbed" by the intensification of fighting in Yemen, especially in populated areas, a spokesman for the UN Secretary General said.
Hospitals and medical personnel are explicitly protected under international humanitarian law and any attack directed against them is a violation of international law, the spokesman added.
The de facto authorities in northern Yemen, dominated by the Houthi rebel movement, blamed the attack on a Saudi-led coalition against the rebels, which has repeatedly pounded the region.
Earlier, Hajjah's provincial health director, Ayman Madhkour, said that 15 people were killed and more than 25 injured in the attack, which he said was carried out by coalition aircraft.
Eyewitnesses told dpa that the strike hit the hospital's emergency department. The victims were all civilians, including many women and children, Madhkour said.
“This is the fourth attack against an MSF facility in less than 12 months.....," said Teresa Sancristóval, MSF's desk manager for the Emergency Unit in Yemen.
MSF is active in 11 hospitals and health centres and provides support to another 18 hospitals and health centres in eight governorates of Yemen
Amnesty international called the strike "an atrocious attack that could amount to a war crime."
“Deliberately targeting medical facilities is a serious violation of international humanitarian law which would amount to a war crime. The circumstances of this attack must be thoroughly and independently investigated,” said Magdalena Mughrabi, Amnesty International’s Deputy Director of the Middle East and North Africa Programme.
Northern Yemen has been hit by frequent airstrikes by the Saudi-led coalition which supports Yemeni President Abd Rabu Mansour Hadi against the mainly Shiite Houthis, who control the capital Sana'a.
On Saturday, 10 children were killed in an attack on a school in neighbouring Saada province, according to MSF. Houthi officials blamed the coalition for that attack.
The conflict, ongoing since early 2014, has intensified since peace talks between Hadi's government and the rebels were suspended on August 6.
The UN secretary general last September called for a halt to the coalition's air campaign, saying it was responsible for most of the conflict's civilian casualties.
Shelling by the Houthis and allied forces on residential areas, especially in the central city of Taiz, has also resulted in considerable civilian casualties.
In June, Ban controversially removed the Saudi-led coalition from the annex of a UN report flagging entities that have killed and maimed children in conflicts.
He subsequently said that the decision had been taken under "undue pressure," including threats from member states to pull funding from various UN agencies in response to the listing.
Earlier on Monday, the coalition announced that it would permit the UN and humanitarian organizations to operate flights into Sana'a airport, whose closure it ordered after the suspension of the peace talks.
An alliance of 12 aid groups had Sunday said the closure of the airport put millions of lives at risk. They called for it to be reopened to commercial as well as aid flights in order to facilitate the arrival of vital supplies and free movement of civilians.
"The closure effectively seals Yemen off from the rest of the world at a time when half the population is malnourished and hospitals urgently require more medicine and medical supplies," Syma Jamil of the Norwegian Refugee Council said.
Yemen, one of the poorest countries in the Arab world, has been devastated by the conflict, which pits the Houthis and their ally, deposed president Ali Abdullah Saleh, against Hadi and a wide range of other forces including local tribes, Sunni Islamists, southern Yemeni secessionists and jihadis.