UN chief Ban Ki-moon on Tuesday denounced strikes that he said were conducted by Saudi-led warplanes on a hospital in Yemen operated by international charity Doctors Without Borders.

Ban "condemns the air strikes by the Saudi-led coalition that hit the Hayadeen Medical Hospital, run by Medecins Sans Frontieres," said a UN statement, using the charity's French name.

The hospital in the rebel stronghold of Saada was hit repeatedly, said the medical aid group, which posted pictures on its Twitter account showing a collapsed roof and rubble.

The building "was hit by several air strikes" on Monday night "with patients & staff inside the facility," the Paris-based MSF tweeted.

"The staff just had time to run off as another missile hit the maternity ward."

There were no deaths, said MSF spokeswoman Malak Shaher.

Amnesty International said the hospital had "more than 20 people inside at the time" of the strikes.

Seven staff members were wounded, said the human rights watchdog, "but could not be taken to another hospital" until the next morning "due to fears of further strikes".

MSF did not immediately confirm the report nor apportion blame.

In the statement, UN Secretary General Ban noted that "several people" were wounded in the strike that destroyed the hospital, and called for an immediate investigation.

Saada is the stronghold of the Iran-backed Huthis who overran the capital unopposed in September 2014 before advancing on several Yemeni provinces.

A Saudi-led coalition of Arab states launched an air campaign against Shiite Huthi rebels and their allies in late March in support of President Abedrabbo Mansour Hadi.

- 'Unlawful attack' -

Amnesty also called for "an urgent, independent and thorough investigation" into the raids, saying that the hospital was hit by "up to six consecutive air strikes".

"The attack on Haydan Hospital appears to have been an unlawful attack causing harm to civilians and civilian objects," said Philip Luther, Amnesty's director for the Middle East and North Africa.

"The consecutive air strikes show deliberate targeting of the medical facility -- this is another sad day for civilians."

Amnesty quoted the hospital's director Ali al-Mughli as saying that the facility was "completely destroyed with the exception of the storage rooms".

The hospital receives injured fighters but "there was no military activity in the hospital at the time of the attack," he was quoted as saying.

In Afghanistan, at least 30 people were killed earlier this month in a US bombing of an MSF hospital in Kunduz.

"We call on all parties to the conflict to respect and protect medical personnel and units and take every precaution to protect civilians caught up in the conflict," said Luther.

- Strikes, clashes in Taez -

Yemen has slid deeper into chaos since the Saudi-led coalition launched its air war on the Huthis and their allies.

Nearly 5,000 people have been killed in the bombing campaign, more than half of the civilians, according to UN estimates, while 80 percent of the population is in dire need of humanitarian aid.

There was more bloodshed in the past two days, particularly in southwestern Yemen where at least 11 civilians were killed in artillery shelling on Taez, a city seen as key to controlling Sanaa.

A health official said that in addition to the civilian deaths, eight were also wounded.

Hashem al-Sufi, a commander of pro-government forces in Taez, accused Huthi rebels and militia supporters of former president Ali Abdullah Saleh of carrying out the shelling.

He also reported that 28 rebels were killed in clashes and coalition air strikes in Taez in the past day and that seven pro-government fighters also died in the battles.

It was not possible to independently verify this toll as the rebels rarely acknowledge their losses.

Loyalists control the centre of Taez, encircled by the Huthi rebels and allied forces loyal to Saleh.

The city, neighbouring Sanaa, has been a key battleground as forces loyal to Hadi seek to regain ground from the rebels.

In July, loyalists backed by the Saudi-led coalition evicted the rebels from five southern provinces and have set their sights on Sanaa.