People move boxes from a damaged truck outside a building hit by a Saudi-led air strike in Sanaa, Oct. 25.

A hospital in north Yemen run by the medical aid group Medecins Sans Frontieres (MSF), was destroyed by a missile strike late Monday, MSF said.The Saudi-led Arab coalition involved in Yemen’s civil war denied that its planes hit the hospital. 

The coalition joined the fighting in Yemen in March to try to restore the government after it was toppled by Iran-allied Houthi forces, but a mounting civilian death toll has alarmed human rights groups.

“Our hospital in the Heedan district of Saada governorate was hit several times. Fortunately, the first hit damaged the operations theater while it was empty and the staff were busy with people in the emergency room. They just had time to run off as another missile hit the maternity ward,” MSF country director Hassan Boucenine told Reuters by telephone from Yemen.

“It could be a mistake, but the fact of the matter is it’s a war crime. There’s no reason to target a hospital. We provided (the coalition) with all of our GPS coordinates about two weeks ago.” He said at least two staff members had been hurt by flying debris.

The attack occurred on Monday night in north Yemen’s Saada province, a region controlled by Houthi forces. The state news agency Saba, run by the Houthis, said other air strikes had hit a nearby girls’ school and damaged several civilian homes.

It was not immediately possible to confirm that report.

MSF said the hospital had been hit by missiles from coalition jets.

Coalition spokesman Brigadier General Ahmed Asseri said in an email that coalition jets had been in action over Saada governorate. But when asked if they had hit the hospital, he said: “Not at all”, and that only an investigation would show the cause of the blasts.

Elsewhere, medical sources and a local official said 13 Houthi fighters and six fighters loyal to Yemeni President Abd-Rabbu Mansour Hadi had been killed on Tuesday in fighting, including coalition air strikes, in the southwestern city of Taiz. Reuters could not independently verify the accounts.

UNICEF said the hospital in Saada was the 39th health center hit in Yemen since March. “More children in Yemen may well die from a lack of medicines and healthcare than from bullets and bombs,” its executive director Anthony Lake said in a statement.

U.N. spokesman Stephanie Dujarric said Ban was calling for “a prompt, effective and impartial investigation in order to ensure accountability” and had urged all parties in the conflict to “immediately cease all operations, including air strikes”.

Twenty-two people including 12 MSF staff were killed when an MSF hospital was hit by an American air strike in Kunduz in northern Afghanistan on Oct. 3.

President Obama apologized for that attack, but MSF continues to call for an independent humanitarian commission to investigate what it calls a war crime.

Seven months of airstrikes in Yemen by Saudi Arabia and other U.S.-allied Gulf Arab countries have yet to loosen the Houthis’ grip on the capital, Sanaa.

The southern port city of Aden is the nominal seat of Hadi’s government, which returned from exile in Saudi Arabia but then mostly went back to Riyadh after a wave of Islamic State attacks in the city three weeks ago.

The United States and Britain are supporting the coalition with intelligence and both are long-time arms suppliers to their Gulf Arab allies.

More than 5,600 people have died in Yemen’s conflict. Shuttle diplomacy by a United Nations envoy has yet to secure a political solution or reduce the intensity of combat.