A Saudi-led coalition air strike killed 13 teaching staff and four children in northern Yemen, in a raid apparently targeting Shiite rebels, medics and witnesses said Thursday.

UNICEF condemned what it called Tuesday's "senseless bloodshed" in Amran province that it said killed 17 civilians and also injured 20 other people.

Saudi Arabia has led an air war against Shiite Huthi rebels and their allies in neighbouring Yemen since March 26.

Five Huthis were also killed in the strike, according to medical sources who gave the same civilian death toll of 17 including four children.

The raid hit a building housing offices for a teachers' union as well as the pro-rebel General People's Congress party during a meeting of Huthi militiamen, witnesses said.

The civilian victims have "paid a most terrible price in the continuing conflict in Yemen", UNICEF said.

"The educators were gathered together to prepare exams for thousands of children who had missed the end of their school year because of this brutal conflict," the agency said.

"Working after hours, they had brought their children with them. Some of whom were playing outside when the bomb exploded," it said.

"A selfless activity turned in a moment into senseless bloodshed."

Nearly 400 children have been killed and over 600 others injured in the past four months, according to UNICEF.

The United Nations says the war in Yemen has killed a total nearly 4,500 people, many of them civilians.

On Wednesday, the top UN aid official strongly criticised Saudi-led air strikes on the Yemeni port of Hodeida, a lifeline for imports of food, medicine and fuel.

"These attacks are in clear contravention of international humanitarian law and are unacceptable," Stephen O'Brien told the United Nations Security Council.