The United Nations humanitarian chief says he is horrified by the attacks on civilians that are taking place in Syria.

Stephen O'Brien tells reporters that "attacks on civilians are unlawful, unacceptable and must stop."

O'Brien spoke in Damascus on Monday at the end of a three-day visit to Syria, during which he met senior officials and visited the central province of Homs.

His comments came a day after government air raids killed at least 96 people in the Damascus suburb of Douma, making it one of the deadliest single incidents since the country's crisis began in March 2011.

O'Brien said: "I am horrified by the total disrespect for civilian life in this conflict."

Syria's conflict has killed more than 250,000 people and wounded at least a million.