Peace talks between the warring parties in Syria, which were scheduled to open Monday, will begin in Geneva on January 29 and will last for six months, the UN's Syria envoy Staffan de Mistura said.

De Mistura told reporters that the start date was pushed back because of a "stalemate" over the make up of the delegations, but that the invitations to the delegates were expected to be sent out on Tuesday, AFP reported on Monday.

"We are going to aim at proximity talks starting on the 29th and ongoing for six months," De Mistura said after US Secretary of State John Kerry said he hoped for "clarity" within 24 to 48 hours on Syrian peace talks that were supposed to have started in Geneva on Monday.

Kerry told reporters on a visit to Laos he agreed with de Mistura that invitations for the talks should not be sent out until "you have pieces lined up," Reuters reported on Monday.

The future of the talks was in the hands of the Syrian parties, he added.

"They have to be serious US. If they are not serious, war will continue. Up to them - you can lead a horse to water; you can't make it drink," Kerry said.

"We have created a framework; the Syrians have the ability to decide the future of Syria," Kerry said.

"We will have to see what decision Staffan makes as to exactly how he is going to begin; but we don't want to decide and have it crumble on day one. It's worth taking a day or two, or three, or whatever," he added.

The talks between the Syrian government and opposition had been due to start in Geneva on Monday, but a Western diplomat said on Sunday they were unlikely to begin before Wednesday.

The Syrian government has said it is ready to attend, but the opposition's High Negotiation Committee, which groups political and armed opponents of President Bashar al-Assad, has said it will not until the government halts bombardments, lifts blockades, and releases detainees - steps mentioned in a UN Security Council resolution last month.

Kerry rejected comments from the lead negotiator of the Syrian opposition, who said on Sunday it was coming under pressure from him to attend the talks.

"I don't know where this is coming from. Maybe it's a pressure thing or an internal political thing, but that is not the situation," he said.

"The position of the United States is and hasn't changed, that we are still supporting the opposition politically, financially and militarily," he added.

Syria's civil war has killed 250,000 people and driven more than 10 million from their homes.