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.
"If there is seriousness in making these talks lead to something substantial, formality won't be that important, we haven't seen any signs of commitment to ceasefire, there should be clarity and agreement on this first," opposition spokesman Yahya al-Aridi told Al Jazeera on Monday.
"Being here, and having all the parties together here, expresses that Russia is serious, but we want to see things happening on ground, that is why things are not quite optimistic for us."
Al Jazeera's Mohammed Jamjoom, reporting from Astana, said the refusal to hold direct talks 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."
READ MORE: Rebels focused on ceasefire at Astana peace talks
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.
Experts say a breakthrough could see some of the armed opposition join the Geneva talks.
"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.

Red lines

The December 30 truce has largely been holding, despite pockets of violence namely in the suburbs of the capital Damascus, where Syrian government forces have been advancing to retake strategic areas.
ASTANA TALKS: WHAT WE KNOW
- The meetings are being sponsored by Syrian government ally Russia and opposition backer Turkey
- Syria's government is sending 10 members, led by UN ambassador Bashar al-Jaafari
- The opposition delegation is made up of about a dozen armed groups and the High Negotiations Committee
- The US is not sending a team, but is being represented by its ambassador to Kazakhstan
- Staffan de Mistura, the UN's Syria envoy, is also attending the talks
Bashar al-Jaafari, the head of the Syrian government delegation, told reporters on the plane to Astana on Sunday that the agenda would focus on strengthening last month's truce.
Jaafari played down Turkey's role as a party to the talks, saying they were between Syrians only.
"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.
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."
READ MORE: In Astana, Russia is 'in full charge' 
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 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.