"The Syrian government delegation has already arrived," Zhainakov said, adding that the delegation was headed by Syrian UN Ambassador Bashar Jaafari.
He added that the delegation of the UN experts had arrived as well, with others to follow "in the coming days."
It has been planned that prior to the talks, in which Russia, Turkey and Iran take part, a meeting between these countries will be held on Tuesday at an expert level.
"De Mistura will arrive tomorrow evening. He will participate in the high-segment part of the talks," the source said.
A source in the Syrian opposition delegation told Sputnik on April 30 that it was most likely that Syria's armed opposition would take part in the next round of Astana talks.
Earlier on Monday, Fatih Hassoun, a representative of the Syrian armed opposition, told Sputnik that Russia had made useful proposals about the observance of the ceasefire regime, about establishment of zones of de-escalation of tensions and about involvement of new international actors in these agreements.
The paper also said that the Russian proposals envisaged possible deployment of guarantor states' armed groups to Syria for ceasefire monitoring.
Astana has already hosted three rounds of talks brokered by Russia, Turkey and Iran, held in January, February and March.