Story highlights Syria has been locked in a civil war since 2011

At least 250,000 people have been killed, says the U.N.'s top humanitarian official

Stephen O'Brien speaks at the end of a three-day visit to Syria

(CNN) The top U.N. humanitarian official said Monday that the conflict in Syria continues to escalate and that he is "horrified" by what he's seen.

Stephen O'Brien spoke to reporters at the end of a three-day visit to Syria.

"I am absolutely horrified by the total disregard for civilian life by all parties in this conflict," he said. "Attacks on civilians are unlawful, unacceptable and must stop. I appeal to every party engaged in violence and fighting to protect civilians and to respect international humanitarian law."

Syria has been locked in a civil war since 2011. At least 250,000 Syrians have been killed, more than a million injured, and almost half of the population have been displaced, according to O'Brien.

Just Sunday, airstrikes from government forces hit the rebel-held town of Douma , killing as many as 82 people and wounding hundreds, the Syrian Observatory for Human Rights and aid groups reported. Activists said at least 250 people were wounded.

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