A rocket fired by Yemeni Houthi forces or allied troops killed at least six civilians when it landed in a crowded market in the southwestern city of Taiz on Friday, medics said.
The medics said children were among the victims of the Katuysha rocket, which was fired from an area controlled by the Houthis and troops loyal to former President Ali Abdullah Saleh.
Forces loyal to Saudi-backed President Abd-Rabbu Mansour Hadi broke a siege imposed by the Houthis on Taiz - Yemen's third-largest city and a cultural center - in heavy fighting in March.
But neither side has been able to extend control over the entire city.
The medics said 18 people were wounded as well as the six killed in the attack, which took place days before the start of Ramadan, the Muslim holy month.
Ezzeldin al-Asbahi, human rights minister for the internationally recognized government based in Aden, put the death toll at nine civilians and said 26 other people were wounded.
"The rocketing of Taiz by the Houthis and Saleh is a proof that the killers are destroying any hope of peace," Asbahi said in a statement.
A Saudi-led alliance intervened in the Yemen conflict in March last year to try to restore Hadi to power after the Houthis, a Shi'ite movement, had taken over the capital Sanaa advanced on his temporary headquarters in Aden, forcing him to flee to Saudi Arabia.
A shaky truce between the Iran-allied Houthis and Hadi loyalists has repeatedly been violated by both sides since it took hold in April before U.N.-sponsored peace talks in Kuwait.
The talks have made little progress toward ending the war that has killed more than 6,200 people and displaced more than 2.5 million.
United Nations Secretary-General Ban Ki-moon on Thursday blamed the Saudi-led coalition of responsibility for 60 percent of child deaths during the war and added it to an annual blacklist of states and armed groups that violate children's rights during conflict.
