The medical charity group, Doctors Without Borders (MSF), said that a projectile hit a hospital it supports in northern Yemen on Sunday, killing four people and injuring 10.
The group added that the attack resulted in the collapse of several buildings of the medical facility in the Razeh district in the Saada province, a stronghold of the Houthi rebels.
Three of the injured are MSF staff, two of them in critical condition, according to the group.
"MSF cannot confirm the origin of the attack, but planes were seen flying over the facility at the time," it added.
"We reiterate to all parties to the conflict that patients and medical facilities must be respected and that bombing hospitals is a violation of international humanitarian law," director of MSF operations, Raquel Ayora, said.
The attack was the third on an MSF health facility in Yemen in the last three months, the group said.
A Saudi-led coalition has been carrying out an air campaign in Yemen against the Iran-allied Houthis since March.
Saada has since experienced intensified coalition bombardment.
Saudi Arabia fears that victory for the rebels in Yemen would give its rival Iran a strategic foothold on the Arabian Peninsula.
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