Syrian rebels have cast doubt on attending Russian-backed peace talks this week, accusing Moscow of failing to get Damascus to fully comply with a ceasefire or take tangible steps to fulfil their demands.

A rebel official, speaking on condition of anonymity, said on Monday that at most a handful of rebels might attend, but only if progress was seen in the next two days.

Russia had so far failed to yield any tangible steps towards full implementation of the ceasefire, humanitarian aid access, or a release of female detainees the rebels had demanded at the first meeting in the Kazakh capital Astana, he added.

"It seems Russian pressure is of no benefit," the official said.

READ MORE: Russia, Turkey, Iran discuss Syria ceasefire in Astana

Kazakhstan said on Saturday it had invited the government and rebels for February 15-16 talks. They attended a similar indirect meeting in Astana last month aimed at shoring up a ceasefire brokered by Turkey and Russia, President Bashar al-Assad's most powerful ally.

"The opposition factions will not attend Astana because the Russian side did not abide by what they agreed to before, during and after Astana to uphold the ceasefire agreement," Mohammad Al Aboud, a second senior rebel official said.

Russia and Turkey, which backs the rebels, have sought to revive diplomacy towards ending the war since the Syrian government and its backers defeated the rebels in Aleppo in December, their biggest defeat of the conflict.

A new round of UN-backed peace talks are due to begin in Geneva next week.

The Syrian government said earlier on Monday it was ready to agree on prisoner swaps with rebel groups, which the opposition wants before any negotiations over Syria's political future.

READ MORE: Astana summit - Opposition sets demands for new talks

Syrian state media cited an official source as saying the government was "always ready" to exchange prisoners in its jails for people "kidnapped by terrorist groups".

A rebel official dismissed the statement as a ruse, saying Damascus had far more detainees than the few the rebels held.

Geneva Talks

The United Nations special envoy for Syria, Staffan de Mistura, sent invitations on Monday for the Geneva talks beginning on February 23, after initial prior consultations beginning on or about February 20, his spokeswoman Yara Sharif said.

The main Syrian opposition body on Sunday approved its delegation to next week's Geneva talks.

This month, in a rare move, the Syrian government and rebel groups swapped dozens of women prisoners and hostages, some of them with their children, in Hama province in northwestern Syria.

Amnesty International said in a report this month that the government had executed up to 13,000 prisoners in mass hangings at a military jail near Damascus. The Syrian justice ministry called the report "devoid of truth".