U.N. envoy Staffan de Mistura struggled Monday to keep peace talks on track in Geneva, as the Syrian government — with Russia's help — launched a major offensive against rebel forces.
U.N. officials said talks that had been scheduled between the U.N. envoy and the Syrian government delegation for Monday were postponed to Tuesday.
Instead, de Mistura met with the opposition for two hours before declaring the talks officially open.
A spokesman for the main opposition group said earlier Monday he had received a "positive response" from de Mistura on opposition demands to halt airstrikes and allow the flow of humanitarian aid to rebel-held areas. An opposition representative described Monday's talks as positive, as well.
But opposition delegates also said escalating violence could hinder the already fragile process.
U.S. Senator Bob Corker, Chairman of the Senate Committee on Foreign Relations, discussed the Syria talks with VOA on Monday, saying he thinks they are starting off in a difficult place.
"I think the opposition that, you know, we communicated some with last week has been put in a very awkward place. The humanitarian things that they had asked for have not occurred."
WATCH: Sen. Bob Corker discusses Syria talks with VOA
He added, "I certainly support diplomatic talks as I think everyone would, but the elements at present do not feel that they are in a proper place.”
Syrian forces, with Russia’s help, have stepped up attacks and launched an offensive Monday north of the city of Aleppo.
The opposition says the government airstrikes are killing civilians and they renewed their threats to leave the talks if the Syrian government does not take steps to show good will.
“You cannot ask the Syrian opposition to engage in any negotiation with the regime under this escalation,” Farah Atassi, a member of the opposition, said in Geneva.
The group boycotted the first day of talks Friday and the only consultations held that day were between the U.N. envoy and the Syrian government delegation. Opposition representatives arrived in Geneva late Saturday after getting assurances from the U.N. and international supporters of the peace process that their demands would be addressed.
A member of the opposition delegation said those assurances had come from U.N. Secretary General Ban Ki-moon and U.S. Secretary of State John Kerry.
In a video statement Sunday, Kerry urged all sides to press on to end to the conflict that has killed 250,000 people, and which he described as "an unfolding humanitarian catastrophe unmatched since World War II."
"I appeal to both sides to make the most of this moment, to seize the opportunity for serious negotiations, to negotiate in good faith with the goal of making concrete measurable progress in the days immediately ahead,'' Kerry said.
Proposal under consideration
Opposition representatives said they did not know how long they will remain in Geneva. Diplomatic sources say the opposition has been considering a proposal from de Mistura that could lead to their staying and participating in indirect negotiations with the Syrian government.
The U.N. envoy's meeting with the opposition Monday and his plan to consult with the Syrian government delegation Tuesday indicated he was making some progress, although difficult and slow, toward establishing an indirect exchange.
Senate correspondent Michael Bowman contributed to this report.