Kerry: 'Clarity' on Syria talks within 48 hours
Keenly awaited peace talks are due to begin on Monday in Geneva but disagreements among the main sponsors and the parties due to attend have disrupted the timetable.
Speaking in Laos, the US secretary of state said he had held conversations with key figures including his French, Turkish, Russian and Saudi counterparts.
Kerry had also spoken to the UN's special envoy to Syria, Staffan de Mistura, "to make sure everybody is on the same page" ahead of the talks.
"We are going to have the meeting and they [the talks] are going to start, we will have clarity I hope within 24 hours, somewhere like that, 48, something pretty soon," he said.
"We are talking about the modalities of the ceasefire, we are talking about the modalities of humanitarian or other confidence-building measures," Kerry said, adding he felt "positive" over the negotiations.
Kerry's efforts are part of the biggest diplomatic push yet to resolve a civil war that has ground on for almost five years.
But representatives of the Syrian opposition have accused him of carrying "clear dictates from Russia and Iran".
The Saturday meeting in Riyadh between Kerry and the head of the Syrian opposition's High Negotiation Council was "catastrophic", a high-level source in the Syrian opposition told The New Arab.
"Kerry carried with him clear dictates from Russia and Iran and threatened the Syrian opposition in the event of non-compliance," added the source.
The source, who asked not to be named, quoted Kerry as saying the Geneva meeting would seek a national unity government rather than a transitional governing body, as the opposition demands.
The source said Kerry told the opposition UN envoy de Mistura should have the right to shape the line-up of the opposition's negotiating delegation, and dismissed the opposition's demands for confidence-building measures - saying they would be subject to negotiations in Geneva rather than being conditions for the talks.
"Kerry said during the meeting Bashar al-Assad should have the right to run in the presidential elections to be held later," further claimed the source.
'US pressure and backtracking'
The lead negotiator in the Syrian opposition said on Sunday the opposition was coming under pressure from Kerry to attend peace talks in order to negotiate over steps including a halt to airstrikes.
The opposition's High Negotiation Council, which groups political and armed opponents of President Bashar al-Assad, has said it will not attend negotiations until the government halts bombardments, lifts blockades, and releases detainees - steps mentioned in a United Nations Security Council resolution passed last month.
Negotiator Mohamad Alloush said Kerry, who met HNC officials on Saturday, had "come to pressure us to forgo our humanitarian rights... and to go to negotiate for them".
"There will be a big response to these pressures," he told Reuters, without giving further details. Asked if the peace talks would go ahead this week, he said "we leave this to the coming hours".
Meanwhile, the government said it was ready to attend the talks. They had been due to begin in Geneva on Monday, but a Western diplomat said earlier on Sunday that they were unlikely to begin before Wednesday.
"The negotiating team is obviously dominated by a certain party from both the political and armed opposition, excluding other parties that include the committee's members and allies," the NCC said in a statement obtained by The New Arab.
"Our executive office upholds the committee's right to active participation in the Geneva talks," the statement added, calling for the talks to be held on the scheduled dates.