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The umbrella body comprising Syria's principal mainstream opposition groups threatened on Saturday to withdraw from peace talks shortly after its delegation arrived in Geneva if the "crimes" of Bashar al-Assad's government persisted.

"If the regime insists on continuing to commit these crimes then the HNC (High Negotiations Committee) delegation's presence in Geneva will not be justified," HNC coordinator Riyad Hijab warned in a statement in Arabic posted online.

"The delegation will inform (UN special envoy Staffan) de Mistura of its intentions to withdraw its negotiating team if the UN and world powers are unable to stop these violations," said Hijab, who was not among those present in Geneva.

It was not clear what has prompted the opposition to make this threat.

But on Saturday, it was announced that sixteen more people have died in the regime-besieged town of Madaya since an aid convoy was allowed to enter the town earlier this month.

On Saturday as well, suspected Russian airstrikes killed at least 10 civilians Saturday in a village held by IS in the eastern Syrian province of Deir ez-Zor.

The Syrian Observatory for Human Rights said three children were among those killed in the raids.

HNC representatives landed in Geneva late Saturday a day after a delegation for President Assad's government arrived and held preliminary talks with de Mistura.

But the body, expected to meet de Mistura on Sunday, is refusing to engage, even indirectly, with the government for now.

This comes as reports emerged the Syrian regime delegation to Geneva includes a senior officer accused of war crimes.

According to images circulated online by activists, the delegation includes State Security Colonel Samer Baridi.

The activists said Baridi was in charge of snipers deployed in the city of Douma at the start of the rebellion, who are accused of killing hundreds of civilians and peaceful activists, as well as assassinating opposition figures.

Syrian opposition leaders are yet to comment on these reports.

Reports emerged the Syrian regime delegation to Geneva includes a senior officer accused of war crimes

HNC demands

The HNC is demanding first that UN Security Council resolutions requiring an end to sieges of more than a dozen Syrian towns are adhered to and that bombardments of civilians cease.

The intra-Syrian negotiations, if they get going, are part of a peace plan set out in November in Vienna by external powers embroiled in the five-year-old conflict, some on different sides.

The process envisions elections within 18 months but leaves unresolved the future of Assad, whose government has been making gains on the ground since Russia began supporting him with airstrikes in September.

Another thorny issue is which rebel groups will be involved in the talks, although all sides agree on the exclusion of extremists from IS and the Nusra Front tied to Al-Qaeda.

Syria's largest mainstream opposition group on Friday had said it would take part in the Geneva "political process", after four days of suspense over whether it would join UN-lead peace talks.

"We have decided to take part in the political process to test the seriousness of the other side through discussions with the UN team to implement international obligations and humanitarian demands," the HNC said in a statement, which came after Saudi Foreign Minister Adel al-Jubeir met with the committee on Friday evening at a Riyadh hotel.

The HNC statement said it has received support "especially... related to the humanitarian situation" from United States Secretary of State John Kerry, foreign ministers of "sisterly countries", as well as from the UN's Syria envoy Staffan de Mistura.

The Saudi foreign ministry later issued a statement saying it welcomed the Committee's decision "to take part in the Geneva negotiations to implement UN Security Council resolution 2254".

The HNC was formed in December when the main Syrian political opposition and armed factions came together in Riyadh for an unprecedented bid at unity, after months of Saudi efforts.