"All parties to this appalling conflict should understand that they will one day be held accountable for violations of international humanitarian and human rights law," he said, the UN News Center reported.

He said that initial reports indicate that at least 30 people were killed, and over 80 injured, among them many women and children, while dozens of tents were destroyed or badly damaged.

"Modern military technology means that there is little room for error," he said, noting that if this "obscene" attack is found to be a deliberate targeting of a civilian structure, it could amount to a war crime.

Read alsoMigrant numbers in Aegean falling – NATO chiefContinued fighting and airstrikes mean that vulnerable, frightened children, women and men have nowhere safe to go. Already this year, thousands of Syrians have been forced to flee their homes, seeking safety from bombs, shelling and other violence, while millions are still trapped without access to food or medical care, he added.

Read alsoIslamic State gunmen partially seize gas field east of HomsInternational humanitarian law clearly sets out the responsibilities of warring parties to protect civilians and to take every possible measure to avoid places where civilians are living and where they are being looked after by humanitarians, he said.