UN Human Rights Commissioner Zeid Ra'ad al-Hussein said on Monday that a peace deal granting amnesty to both sides of the Syrian conflict should not include war crimes and crimes against humanity.

"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," he said.

The siege of more than 15 towns, including Madaya, by government forces and rebel groups resulting in starvation is "not just a war crime but a crime against humanity if proven in court," the UN human rights chief added.

In January, the UN secretary-general made a similar statement after news emerged that hundreds of people starved to death in the city of Madaya. "Starvation as a weapon is a war crime," Ban Ki-moon said.

Al-Hussein's statement in Geneva comes as peace talks aiming to secure a political end to the nearly five-year conflict in Syria have been stalled by a reluctant opposition that wants their demands met before entering formal negotiations with President Bashar al-Assad's government.

'Humanitarian issues'

"We only came to Geneva after written commitments on the fact that there should be serious progress on the humanitarian issues," said Basma Kodmani, a spokeswoman for the High Negotiations Council, a Saudi-backed umbrella coalition of rebel groups vying for al-Assad's ouster.

"We are here for political negotiations, but we cannot start those until we have those gestures," Kodmani added.

The HNC has called for the government to stop bombing civilians, end the siege on several Syrian towns and release political prisoners.

Indeed, Syrian forces are continuing their unrelenting assault, backed by Russian air power. More than 3,000 Turkmen and Arabs have crossed into Turkey in the past three days - forced by advancing Syrian troops.

And many more are expected to cross in the coming days as a displaced persons camp is being evacuated inside Syria.

"After the attacks have spilled over to Yamadi camp, the first group of 731 migrants, mostly babies, children, women and the elderly, have entered our country," a Turkish aid agency, AFAD, said in a statement.

More broadly, the UN says about 13,000 have been displaced along the Turkish border over the past 12 months alone. And in southern Syria another 36,000 have been forced to flee by advancing government troops seeking to retake the strategic town of Sheikh Maskin.

However, the government delegation said the opposition was "not serious" following the appointment of Mohamed Alloush - a political leader of the Saudi-backed Army of Islam rebel group deemed a "terrorist" group by Moscow and Damascus - as the HNC's chief negotiator.

UN Syria envoy Staffan de Mistura is expected to meet with the rebel delegation on Monday.

More than 250,000 people have been killed and half the population displaced since the conflict erupted in 2011 after government forces fatally quashed peaceful demonstrations calling for al-Assad to step down.

ls/jil (Reuters, AFP, AP)