Secretary-General Ban Ki-moon has condemned the attacks in Yemen's Taiz where heavy weapons, including rockets, mortars and artillery were used.
Rocket fire reportedly hit a busy market in the city.
A year of conflict between forces loyal to the President and Houthi rebels has displaced millions of people in Yemen.
Jocelyne Sambira reports.
Attacks directed against civilians and populated areas, including markets, are strictly prohibited, UN Secretary-General Ban Ki-moon warned warring parties.
In a statement released on Saturday, he called for an independent investigation.
Targeting civilian areas is a violation of international humanitarian law, Mr Ban underscored.
A nation-wide cease-fire went into effect on 10 April.
But Taiz continues to pay a heavy toll in civilian losses despite the cessation of hostilities and it's particularly regrettable, the UN chief said.
Calling on those participating in the peace talks in Kuwait to negotiate in good faith, he also asked them to work with his Special Envoy, Ismail Ould Cheikh Ahmed, to bring a peaceful end to this devastating conflict.
In this same spirit, Mr Ban also urged the importance of the immediate release of prisoners and detainees.
Jocelyne Sambira, United Nations.
Source: United Nations Radio