More than 15 civilians were killed and 20 more injured in attacks on minibuses in Yemen's Hudaydah province, a United Nations coordinator said Sunday.

"This is a horrific incident," Lise Grande, the U.N. Humanitarian Coordinator for Yemen, said in a statement.

"The United Nations agencies working in Yemen unequivocally condemn the attack on civilians and extend our deepest condolences to the families of the victims," she added.

The attacks reportedly struck the minibuses as they were traveling in Jabal Ras district in Hudaydah on Saturday.

"Under international humanitarian law, parties to the conflict are obliged to respect the principles of precaution, proportionality and distinction," Grande said.

"Belligerents must do everything possible to protect civilians — not hurt, maim, injure or kill them," she stressed.

According to U.N. estimates, more than 170 people have been killed and 1,700 others injured, while more than 425,000 people have been forced to flee their homes in Hudaydah since June.

Impoverished Yemen has remained wracked by violence since 2014, when Shia Houthi rebels overran much of the country, including capital Sanaa.

The conflict escalated in 2015 when Saudi Arabia and its allies launched a massive air campaign in Yemen aimed at rolling back Houthi gains.

The ongoing violence has devastated Yemen's basic infrastructure, prompting the U.N. to describe the situation as "one of the worst humanitarian disasters of modern times."