UNITED NATIONS, April 8 (Xinhua) -- As many as 11 civilians, including five students, have been killed in clashes in the capital city of Sanaa on Sunday, a UN spokesman said Monday, calling for safety for civilians.

"Preliminary reports from our team in Yemen indicate that as many as 11 civilians, including five students, were killed and scores of civilians injured in Shu'aub District in Sanaa City," said UN spokesman Stephane Dujarric.

Dujarric said his office sent messages via social media Sunday evening "about our concern at the deaths in Sanaa and called on all parties to exercise utmost restraint and ensure the safety of all Yemenis."

Lise Grande, the UN Humanitarian Coordinator for Yemen, said that "these are terrible, senseless deaths and injuries and we offer our deep condolences to the families of the victims," Dujarric said. "She said that protecting people and protecting civilian infrastructure (are) core principles of international humanitarian law."

During 2018, humanitarian organizations reported an average of 45 incidents of armed violence each week in Yemen, the spokesman said. Thousands of civilians were killed last year, including more than 950 children.

For more than four years, Yemen has been grappling with a brutal conflict between supporters of Yemeni President Abd-Rabbu Mansour Hadi and Houthi rebel groups. The war has plunged the Arab country into what the UN considers to be the world's worst humanitarian crisis, with four out of five Yemenis (24.1 million people) in need of some form of humanitarian aid and protection.