The Saudi-backed Syrian opposition has ruled out even indirect negotiations with Damascus before steps including a halt to Russian air strikes, contradicting US Secretary of State John Kerry's insistence that talks will begin next week.

With the five-year-old Syrian war showing no signs of ending, it looks increasingly uncertain that peace talks will begin as planned on January 25 in Geneva, partly because of a dispute over the composition of the opposition delegation.

Peace efforts face huge underlying challenges, among them disagreements over President Bashar al-Assad's future and tensions between Saudi Arabia and Iran. Russia on Friday denied a report that President Vladimir Putin had asked Assad to step down last year.

The Syrian government has said it is ready to take part in the Geneva talks on time. The office of UN envoy Staffan de Mistura said he was still aiming "at rolling out the talks" on January 25, and would be "assessing progress over the weekend".

Russia said the talks could be delayed until January 27 or 28 because of the disagreement over who would represent the opposition.

George Sabra, a senior opposition official, said the obstacles to the talks were still there, reiterating demands for the lifting of blockades on populated areas and the release of detainees, measures set out in a December 18 Security Council resolution that endorsed the peace process.

"There must be a halt to the bombardment of civilians by Russian planes, and sieges of blockaded areas must be lifted," said Sabra, who was this week named as a senior member of the opposition delegation to any talks. "The form of the talks does not concern us, but the conditions must be appropriate for the negotiations," he told Reuters.

Sabra's opposition council, the High Negotiations Committee (HNC), was formed in Saudi Arabia last month. It groups Assad's political and armed opponents, including rebel factions fighting Damascus in western Syria.

The west is the main theatre of the war between rebels and Damascus, whose military position has been bolstered since September by Russian warplanes and Iranian ground forces.

Russian warplanes continued to bomb many parts of western and northern Syria on Friday, particularly Latakia province, where the government is pressing an offensive against rebels, the Syrian Observatory for Human Rights reported.

Pro-government forces captured a dam 10 km (6 miles) from the town of Salma, seized last week in one of the most significant gains since Russia intervened. "They've tightened their stranglehold on (rebel) fighters in the Latakia countryside," Observatory director Rami Abdulrahman said.

Later in the day they recaptured another five villages, all in hills overlooking insurgent positions, he said, describing it as a "strategic advance" towards the Turkish border.

Air strikes also hit areas in the east near where government forces have been fighting against Islamic State, which controls most of the province. Raids believed to be either Russian or Syrian killed 30 civilians near Deir al-Zor city, the Observatory said.

HIJAB TO MEET KERRY

While rebels have received military support from Assad's foreign enemies, states including Saudi Arabia, Turkey and Qatar and the United States, their calls for more effective weapons including anti-aircraft missiles have gone unanswered.

One of the biggest rebel factions in the HNC, Jaysh al-Islam, said the opposition was facing "many pressures" to make concessions but credited Saudi Arabia, Turkey and Qatar for helping it to "overcome these pressures".

The lead negotiator picked by the HNC for the hoped-for negotiations is a Jaysh al-Islam member, another potential complication facing the talks because Russia says it is a terrorist group.

HNC chair Riad Hijab is due to meet Kerry on Saturday and "all the matters will be tabled clearly", Sabra said.

A senior US State Department official confirmed Kerry was likely to meet Hijab to "check the tyres on the way forward (on talks)... that will hopefully kick off next week."

Russia views the HNC as a Saudi attempt to dictate who represents the opposition. The HNC has said it will not join any negotiations if a third party attends, rejecting Russia's bid to expand the opposition delegation to include the Kurdish PYD and others.

The Kurds control vast areas of northern and northeastern Syria where they have set up an autonomous administration which they say should be a model for settling the Syrian conflict.

The opposition accuses the Kurds of cooperating with Damascus, a charge they deny. One opposition official said on Thursday the Kurds should attend on the government side.

Syrian Kurdish leader Saleh Muslim told Reuters that the Syrian Kurds must be represented at peace talks or they will fail. He also accused Jaysh al-Islam of fostering the "same mentality" as al Qaeda and Islamic State.

"If there are some parties that are effective in this Syria issue who are not at the table, it will be the same as what happened in Geneva 2," Muslim, co-chair of the Kurdish Democratic Union Party (PYD) told Reuters, referring to failed negotiations in 2014.

Jaysh al-Islam last year backed away from hardline Islamist rhetoric heard from it earlier in the conflict, saying Syrians should be free to pick their form of government and Alawites were part of the Syrian nation.

The Kremlin on Friday rejected a report that an envoy for Putin asked Assad to step down last year, TASS news agency reported.

The Financial Times had earlier reported that the head of Russian military intelligence agency travelled to Damascus at the end of last year to ask Assad to step down only to be angrily rebuffed.