DUBAI - Aid agency Doctors Without Borders condemned a missile strike on one of its clinics Sunday in northern Yemen, saying it had killed at least four people and wounded 10 others.

Three MSF staff members are among the wounded and two are in "critical condition", the agency said in a statement, adding that toll could rise as several buildings have collapsed. "The numbers of casualties could rise as there could still be people trapped in the rubble," MSF added.

The missile struck the medical facility in the Razeh district of Saada province, the agency said. All staff and patients were evacuated, with patients being transferred to another MSF-supported hospital in Saada, it said. MSF could not specify whether the medical facility was hit in an air strike by the Saudi-led coalition or by a rocket fired from the ground. MSF director of operations Raquel Ayora denounced the missile strike and repeated that the agency constantly shares the coordinates of its medical facilities with warring factions in Yemen.

"All warring parties are regularly informed of the GPS coordinates of the medical sites where MSF works," said Ayora. "There is no way that anyone with the capacity to carry out an airstrike or launch a rocket would not have known" that the hospital was a functioning health facility supported by MSF, Ayora said.

"We strongly condemn this incident that confirms a worrying pattern of attacks to essential medical services and express our strongest outrage as this will leave a very fragile population without healthcare for weeks," said Ayora." "Once more it is civilians that bear the brunt of this war," she added.

Meanwhile, the Saudi-led coalition bombing rebels in Yemen denied Sunday renewed accusations of dropping cluster munitions in the country after UN chief Ban Ki-moon said their use may be a "war crime".

Coalition spokesman Brigadier General Ahmed al-Assiri told AFP that the alliance "denies using cluster bombs in Sanaa", the Yemeni capital.

He was responding to a report issued Thursday by the US-based Human Rights Watch (HRW), which quoted Sanaa residents describing a January 6 attack consistent with airdropped cluster bomb use.

Assiri said the coalition has admitted to previously using CBU-105 cluster bombs against rebel vehicles. "Now they don't have any more vehicles so we don't use it," he said.

He described HRW's report as "very weak", adding that "they didn't show any evidence." Assiri said the coalition didn't possess the specific munition detailed in the report, adding that 90 percent of coalition operations in Sanaa are directed against Scud missile launchers.

"You cannot use a cluster bomb against Scud launchers," Assiri said. "We have nothing to hide." There has been widespread international concern about the high number of civilian casualties in Yemen.

The Saudi-led coalition has been supporting Yemeni forces since March against Iran-backed Huthi rebels and their allies, who seized territory from the internationally-recognised government.

Cluster bombs contain multiple submunitions, which sometimes do not explode and become de facto landmines that can kill or maim long after they were dropped.

The weapons are prohibited by a 2008 treaty adopted by 116 countries, but not by Saudi Arabia, its coalition partners, or the United States.

On Friday UN chief Ban said he had received "troubling reports" of cluster bomb attacks in Sanaa. "The use of cluster munitions in populated areas may amount to a war crime due to their indiscriminate nature," he said in a statement.

The office of the UN high commissioner for human rights said last week that its staff in Yemen had found remnants of 29 cluster bombs during a field visit to Yemen's Haradh district.

In its report, HRW showed a picture of what it said were BLU-63 submunitions from a CBU-58 cluster bomb. Assiri called the report unprofessional and said the rights group did not contact the coalition.

"They collect information, I think, from the Huthi side," he said, referring to rebels. "Showing this kind of photo doesn't mean it is in Sanaa (and) doesn't mean it is from the coalition."

Assiri said the coalition has been targeting missile launchers which have fired "indiscriminately" on the Saudi border regions of Jazan and Najran.

Around 90 civilians and soldiers have died from shelling and skirmishes in Saudi border regions since March when the coalition began air and ground action in Yemen.

More than 5,800 people have been killed in Yemen since March, about half of them civilians, according to the United Nations.