Jamie McGoldrick's yesterday's comments at a Sanaa news conference came one day after airstrikes by a Saudi-led and US-backed coalition targeted a busy market in a northern region controlled by Shiite Houthi rebels, killing and wounding dozens.
The Houthi-controlled state news agency, SABA, said at least 65 people were killed and 55 wounded, including women and children.
The airstrike on the market in the city of Mastaba, in Hajja province northwest of Sanaa, was the latest in a series of similar bombings by the coalition that have killed hundreds of civilians. The market serves tens of thousands of people and was struck during the busiest time of day. Witnesses said there were no military targets nearby.
Yemen's war pits the Houthis and troops loyal to a former president against the internationally-recognized government and an array of allied forces. A Saudi-led coalition entered the war on the government's side a year ago. The fighting has killed more than 6,200 civilians and displaced millions, pushing the Arab world's poorest country to the brink of famine.
The UN and others have also blamed the Houthis and their allies for civilian casualties, saying they frequently shell residential areas.
McGoldrick urged the warring parties to return to the negotiating table, saying military action was unlikely to end the conflict.
