The international aid group said several nighttime strikes hit the small hospital in the Haydan district of Saada province, injuring one staff member.

"This attack is another illustration of a complete disregard for civilians in Yemen, where bombings have become a daily routine," Hassan Boucenine, the organization's head of mission in Yemen, said.

Saudi Arabia and a group of mainly Gulf allies have been carrying out airstrikes against the Shiite Houthi rebels since March, after the group overran the capital Sanaa and forced the internationally-recognized Yemeni government to flee. Saada, a main Houthi stronghold, has been the target of particularly intense bombing from Saudi-led forces.

Doctors Without Borders, also known by its French acronym MSF, demanded an explanation from the Saudi-led forces. It said the hospital's GPS coordinates were regularly shared with the coalition, and that its roof was clearly identified with its logo.

But, according to Reuters, coalition spokesman Brigadier General Ahmed Asseri denied Saudi planes had hit the hospital.

The clinic is the only source of medical help for the 200,000 people who live in the northwestern region.

United Nations Secretary-General Ban Ki-moon condemned the bombing and in a statement urged all parties in Yemen to "immediately cease all operations, including airstrikes."

Amnesty International called for an independent investigation into the strike, saying it could amount to a war crime.

The office for the UN high commissioner for human rights said Tuesday that some some 2,615 civilians have been killed in Yemen violence over the last six months. OHCHR spokesman Rupert Colville said about two-thirds of those deaths were caused by airstrikes, and the rest by Houthi rebels and their allies. Thousands of Yemenis are also in dire need of humanitarian aid.

The strike on the Saada clinic follows a raid by US gunships on an MSF hospital on October 3 in the Afghan city of Kunduz. At least 30 people died in that attack, which was later described as a mistake, prompting an apology from US President Barack Obama. MSF has repeatedly called for an independent investigation, saying the strike constituted a war crime.

nm/bw (Reuters, AP, AFP, dpa)