An airstrike hit a hospital supported by Doctors Without Borders in northern Yemen on Monday, the international aid group said, killing and wounding an unknown number of people.

The group, known by the French acronym MSF, said the strike hit near the Houthi rebel stronghold of Saada, where teams were still attending to the wounded. Yemeni security and medical officials, who spoke on condition of anonymity because they weren't authorized to brief the media, say the strike killed and wounded some 20 of the hospital's staff and patients.

The conflict in Yemen pits an internationally-recognized government backed by a Saudi-led coalition against the Shiite rebels, who captured the capital, Sanaa, in September 2014. The Saudi-led coalition has been carrying out airstrikes in Yemen since March 2015.

On Saturday, an airstrike on a school killed at least 10 children and wounded dozens more, Yemeni officials and aid workers said. The Saudis denied hitting a school, saying the target was a Houthi training camp and accusing the rebels of recruiting children.

Earlier this month, the coalition acknowledged that it had committed "unintended bombings" that caused civilian casualties during its nearly 18-month air campaign, blaming U.N. agencies for not coordinating with it.

Saudi government spokesman Mohammed al-Mansour, in comments published by the Saudi Press Agency, chronicled eight incidents that rights groups said killed hundreds of civilians.

MSF, which operates in conflict zones around the world, has had a number of its facilities attacked over the past year.

The group said in May that at least 100 staff members, patients and caretakers were killed, and another 130 were wounded, in aerial bombing and shelling attacks on more than 80 MSF-supported and run health structures in 2015 and early 2016.

A U.S. aerial attack on an MSF-run hospital in Afghanistan last October killed 42 people. The U.S. government has apologized for the attack and paid compensation to survivors and the families of those killed.

U.N. deputy spokesman Farhan Haq said the U.N. is trying to gather details on Monday's attack.

"We have condemned all attacks on medical facilities by any side and we would similarly condemn any attack that causes the sort of damage that we've heard reported in this attack on an MSF facility," Haq said.

He reiterated Secretary-General Ban Ki-moon's condemnation Sunday of the airstrike on a school in Saada governorate in north Yemen on Saturday that killed at least 10 children and injured many more.

In that statement, Ban expressed dismay that civilians, including children, "continue to bear the brunt of increased fighting and military operations in Yemen" and reiterated that there is no military solution to the crisis, his spokesman said.