BEIRUT/GENEVA (Reuters) – The Syrian military denied it had conducted air strikes on camps near the Turkish border on Thursday which killed at least 28 people, but the U.N. human rights chief said initial reports suggested a government plane was responsible.

The death toll from attack on the camp for internally displaced people near the town of Sarmada included women and children, the Syrian Observatory for Human Rights said, and could rise further because many people were seriously wounded.

“There is no truth to reports … about the Syrian air force targeting a camp for the displaced in the Idlib countryside”, the Syrian military said in a statement on Friday carried by state media.

U.N. High Commissioner for Human Rights Zeid Ra’ad al Hussein said the attacks were almost certainly a deliberate war crime.

“Given these tent settlements have been in these locations for several weeks, and can be clearly viewed from the air, it is extremely unlikely that these murderous attacks were an accident,” Zeid said in a statement.

“My staff, along with other organizations, will leave no stone unturned in their efforts to research and record evidence of what appears to be a particularly despicable and calculated crime against an extremely vulnerable group of people,” he said.

“Initial reports suggest the attacks were carried out by Syrian Government aircraft, but this remains to be verified.”

Footage shared on social media showed rescue workers putting out fires which still burned among charred tent frames, pitched in a muddy field. White smoke billowed from smoldering ashes, and a burned and bloodied torso could be seen.

Sarmada lies about 30 km (20 miles) west of Aleppo, where a cessation of hostilities brokered by Russia and the United States had brought a measure of relief on Thursday.

Zeid said most of the people in the camps had been forced to flee their homes in Aleppo in February because of sustained aerial attacks there.

He said he was also alarmed about the situation in Syria’s Hama central prison, where detainees had taken control of a section of the prison and were holding some guards hostage.

“Heavily armed security forces are surrounding the prison and we fear that a possibly lethal assault is imminent. Hundreds of lives are at stake, and I call on the authorities to resort to mediation, or other alternatives to force,” Zeid said.

He urged governments on the U.N. Security Council to refer Syria to the International Criminal Court so that there is “a clear path to punishment for those who commit crimes like these”.

(Reporting by John Davison in Beirut and Tom Miles in Geneva; Editing by Gareth Jones and Dominic Evans)