A delegation of Syrian rebels attending a new round of talks in Kazakhstan's capital will not hold direct talks with representatives of the government, according to opposition sources.

The meetings in Astana, organised by Russia and Turkey, are aimed at strengthening a shaky ceasefire that has largely held despite incidents of violence across Syria.

Opposition spokesman Yahya al-Aridi said the rebels had backed out of the first session of face-to-face negotiations on Monday, accusing the government of Bashar al-Assad of violating the December 30 truce deal brokered by Russia and Turkey.

"If there is seriousness in making these talks lead to something substantial, formality won't be that important," Aridi told Al Jazeera.

READ MORE: Rebels focused on ceasefire at Astana peace talks

"We haven't seen any signs of commitment to the ceasefire; there should be clarity and agreement on this first."

But Bashar al-Jaafari, the head of the Syrian government delegation, also accused rebels of not keeping their end in the ceasefire deal - particularly in Wadi Barada, a strategic area in the Damascus suburbs and home to a major water facility.

Speaking to reporters in Astana, Jaafari repeatedly referred to the rebel delegation as representatives of "terrorist armed groups" and said the agenda for the talks is "not ready yet".

Al Jazeera's Mohammed Jamjoom, reporting from Astana, described the start of the talks as "rocky", saying the lack of direct negotiations raised concerns about the effectiveness of the meeting.

"It is making diplomatic sources wonder how effective these talks will be, because these meetings really hinged on the idea that the rebels and government representatives will meet face-to-face," Jamjoom said.

"At this stage, there is a lot of concern behind scenes about how these talks will go."

Organisers have played down expectations of a breakthrough, with Numan Kurtulmus, Turkey's deputy prime minister, saying the prospect of an immediate solution was still distant.

"There are parties that have been at war with each other for six years around the table at Astana," he told journalists on Monday.

"A solution in one or two days should not be expected."

The negotiations in Kazakhstan, which are expected be over by midday on Tuesday, are likely to be followed by United Nations-mediated diplomatic talks in Geneva, Switzerland, on February 8.

"The talks in Astana are not an alternative to the Geneva talks next month, but are an additional step," Roman Vassilenko, the Kazakh deputy foreign minister, said.

Ceasefire, aid

The December 30 truce has largely been holding, despite pockets of violence, specifically in the suburbs of the capital Damascus, where Syrian government forces have been advancing to retake strategic areas.

Before the talks, Jaafari had played down Turkey's role as a party to the talks and said the agenda would focus on strengthening last month's truce.

"Turkey is violating Syrian sovereignty, so there is no Syrian-Turkish dialogue," he said, a reference to Turkish support for anti-Assad armed groups in the north of Syria.

READ MORE: In Astana, Russia is 'in full charge'

Syrian opposition officials also said they were focused on securing the current ceasefire, as well as getting humanitarian aid to people living under siege.

"The besieged areas should be relieved from the torture," Aridi told Al Jazeera on Monday.

"Aid should reach these besieged areas and the principle of kneel or starve to death should also be humanely removed."

When asked whether the rebels would sign an agreement to continue negotiations in February's meeting in Geneva, Aridi replied: "It depends on the productivity and fruitfulness of these [Astana] talks. If they are quite successful, there could come a product, a political one, that could be used in the Geneva talks."

Although Ankara and Moscow have backed opposing sides of Syria's nearly six-year conflict, they have worked hand-in-hand in recent weeks to try to secure an end to the war.

The US Trump administration was invited to participate in the talks, but did not send a delegation.

Washington will instead be represented by its ambassador to Kazakhstan, according to the US state department.

Staffan de Mistura, the United Nations special envoy for Syria, will also be attending the talks to play a supportive role.

UN envoy for Syria, Staffan de Mistura, on Sunday hailed the talks as a "good initiative", in comments carried by Russian news agencies.

France and Britain will also be represented at the ambassador level, according to a European diplomatic source.

Hundreds of thousands of Syrians have been killed during the war, which initially started when largely unarmed anti-government uprisings against Assad erupted in 2011.

More than 12 million people, approximately half of the country's prewar population, have also been displaced over the course of the war.

Additional reporting by Zena Tahhan and Dylan Collins in Astana.