Starvation of Syrian civilians is a potential war crime and crime against humanity that should be prosecuted and not covered by any amnesty linked to ending the conflict, the top United Nations human rights official said on Monday.

Zeid Ra'ad Al Hussein, speaking to a news briefing in Geneva as Syria peace talks were being held said: "In the case of Syria, we are there to remind everyone that where there are allegations that reach the threshold of war crimes or crimes against humanity that amnesties are not permissible."

Pointing to what he said was the starvation of people in the town of Madaya, and the siege of 15 other towns and cities in Syria, he said this was "not just a war crime but a crime against humanity if proven in court."

"We estimate that tens of thousands are held in arbitrary detention and clearly they need to be released," Zeid added.