BEIRUT/GENEVA (Reuters) - The Syrian military denied on Friday it had conducted air strikes on a camp near the Turkish border that killed at least 28 people, but a top U.N. official said initial reports suggested a government plane was responsible for the "murderous attacks".

U.N. High Commissioner for Human Rights Zeid Ra'ad al Hussein said Thursday's attacks were almost certainly a deliberate war crime. France called them a "revolting and unacceptable act that could amount to a war crime or crime against humanity".

The Syrian Observatory for Human Rights said women and children were among those killed in the attack on the camp near the town of Sarmada, which sheltered people fleeing the five-year civil war. The monitoring group said the death toll could rise further because many people were seriously wounded.

In a statement published by state media, the Syrian military said: "There is no truth to reports ... about the Syrian air force targeting a camp for the displaced in the Idlib countryside."

Syria's ally Russia said none of its aircraft had flown over the camp. It said militants from the al Qaeda-affiliated Nusra Front might have deliberately or accidentally fired on it.

U.N. rights chief Zeid said in a statement: "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."

He said his organization and others would "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."

Initial reports suggested the attacks were carried out by Syrian government aircraft, but this remained to be verified, he added. He urged governments on the United Nations Security Council to refer Syria to the International Criminal Court so that there would be "a clear path to punishment for those who commit crimes like these".

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CHARRED TENTS

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.

(Reporting by John Davison in Beirut, Tom Miles in Geneva and Alexander Winning in Moscow; Editing by Mark Trevelyan)