Four people were killed when a missile hit a hospital supported by Medecins Sans Frontieres (MSF) in northern Yemen, the aid agency says.
MSF said the strike in Saada province, a Houthi rebel movement stronghold, had injured a further 10 people.
The Houthis are fighting the government and its allies, including Saudi Arabia.
MSF said it was unclear whether the hospital was hit in an air strike by warplanes of a Saudi-led coalition, or by a rocket fired from the ground.
But it added that planes were seen overhead at the time one projectile flattened hospital buildings, and at least one other landed nearby.
The agency said three of those injured in the strike were its staff, two of whom were in a critical condition.
It condemned the strike, saying: “This is the third severe incident in the last three months. Our teams struggle on a daily basis to ensure the respect of health facilities… we reiterate to all parties to the conflict that patients and medical facilities must be respected.”
Another MSF hospital in Sadaa was destroyed last October, while a clinic in Taiz was hit last month. Both strikes have been attributed to coalition aircraft.
Last October, US aircraft bombed an MSF hospital in the Afghan city of Kunduz, causing heavy loss of life.
Meanwhile, US-based campaign group Human Rights Watch (HRW) has accused the Houthi rebels of arbitrarily detaining their opponents in the capital, Sanaa, which they have held for more than a year.
HRW says it documented the detention of 35 people last autumn, most of whom remain in custody.
It also reports a Yemeni legal group’s assertion that it is working on behalf of more than 800 detainees and individuals who have disappeared, most of them from a political party called Islah which opposes the Houthis.
HRW says the arrests have generated a climate in which politicians, lawyers and journalists say they have never been more afraid.
At least 2,795 civilians have been killed in Yemen since last March, when the Saudi-led coalition began a military campaign to restore the government and drive back the Houthis and allied security personnel loyal to ex-President Ali Abdullah Saleh.
In the past six months, coalition and pro-government forces have retaken Aden, but the rebels still control the capital Sanaa.
The already dire humanitarian situation has also deteriorated, with more than 21 million people – four-fifths of the population – now requiring aid.