The planned face-to-face talks would be the first time armed rebel groups have negotiated with President Bashar al-Assad's regime.
Scheduled to begin at 0800 GMT, the planned face-to-face talks would be the first time armed rebel groups have negotiated with President Bashar al-Assad's regime since the conflict erupted in 2011.
The talks have been welcomed by all parties in the war, but the two sides have arrived in Kazakhstan with apparently divergent ideas on their aim, and officials have cast doubt on whether they will in fact sit down at the same table.
Just before the talks were set to begin, opposition spokesman Yehya al-Aridi told AFP that the opposition will "participate in the talks but the first negotiating session will likely be in separate rooms".
Kazakh deputy foreign minister Roman Vasilenko told reporters Monday morning that the format was still under discussion.
Rebel groups have said the meeting will focus on bolstering a frail nationwide ceasefire brokered last month by opposition ally Turkey and regime backer Russia.
But Assad has insisted that rebels lay down their arms in exchange for an amnesty deal, and called for a "comprehensive" political solution to a conflict that has killed more than 310,000 and displaced more than half of Syria's population.
Organised by Turkey, Russia and Iran, the talks come a month after the regime recaptured rebel areas of Aleppo, scoring its biggest victory since the war began.
Chief opposition negotiator Mohammad Alloush arrived in Astana on Sunday accompanied by around a dozen rebel figures, AFP correspondent said.
"This is not a replacement for the Geneva process," rebel negotiator Fares Buyush told AFP, referring to the UN-hosted political negotiations set to resume in the Swiss city next month.
Delegation spokesman Osama Abu Zeid said the rebels were concerned with "more than just a ceasefire".
"The issue is putting monitoring, investigation, and accountability mechanisms in place," he told AFP. "We want these mechanisms so that this doesn't play out over and over."
Previous efforts at securing a long-term ceasefire in Syria have faltered, with both sides trading accusations over violations.
The 10-member government delegation, headed by UN ambassador Bashar al-Jaafari, also arrived on Sunday, Syrian state television reported.
Syrian state media reported that the regime had met the Iranian delegation, as well as the UN's Syria envoy Staffan de Mistura on Monday ahead of the talks to discuss their positions.
Jaafari initially dismissed the possibility of any government-level meeting with rebel backer Turkey.
But state television later quoted him as saying the regime "will participate in any meeting that serves Syrian national reconciliation".
The regime's objectives include reaching "common ground" with other participants, Syrian state news agency SANA quoted Jaafari as saying.
The regime will also seek to "consolidate the cessation of hostilities" and separate the rebels from the Islamic State group and former Al-Qaeda affiliate Fateh al-Sham Front, he said.
As the sides headed to Astana on Sunday, three regime air strikes killed nine civilians in rebel-held areas in the central Syrian province of Homs, a monitoring group said.
The talks will be held in Astana's luxury Rixos President Hotel, where staff members on Sunday set up a single large table in a conference room under blue banners bearing the hashtag #AstanaProcess.
Although Russia and Turkey 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 brutal war.
The rapprochement, which saw Russia and Turkey conduct their first joint air strikes against IS targets in Syria last week, has come to fill the vacuum left by Washington's disengagement from the conflict in recent months.
US President Donald Trump's administration was invited to participate in the talks but did not send a delegation.
Washington will instead be represented by its ambassador to Kazakhstan, the State Department said, while a European diplomatic source said France and Britain would also be represented at the ambassador level.
Experts say a breakthrough could see some of the armed opposition join next month's Geneva talks.
"Nearly six years of war demonstrates there is no shortcut to ending it," a Western diplomat told AFP.
"A genuine transition in Syria first means building confidence on the ground. That is what the opposition have demanded and it's not so much to ask."