The UN special envoy for Syria, Staffan de Mistura, has sent out invitations to the Syrian participants of the Geneva talks scheduled for Friday.
Preparations for the talks have been beset by problems including a dispute over who should represent the opposition.
The UN did not say on Tuesday who had been invited or how many groups might participate.
De Mistura said in a news conference on Monday that the talks will take begin in Geneva on January 29 and are expected to last for six months.
He said the talks will push for a nationwide ceasefire for all parties other than the Islamic State of Iraq and the Levant (ISIL) and al-Nusra Front armed groups.
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After the invites were sent out, conflicting reports emerged of which groups and individuals had finally been invited.
One contentious issue has been whether or not the powerful Kurdish faction that controls wide areas of northern Syria would be invited.
Russia, an ally of Syria's president, has sought to expand the opposition delegation to include the Kurdish Democratic Union Party (PYD), as well as other individuals it believes represents Syrian society.
The Syrian opposition platform created in Saudi Arabia last month, who were meeting in Riyadh on Tuesday to decide whether to send their delegation to the talks, say the PYD should be part of the government delegation.
Al Jazeera's James Bays, reporting from Geneva, said that the PYD leader was not invited to the talks.
"We spoke to the PYD leader, Salem Muslim, who is in Switzerland but said he was not invited to the talks," Bays said.
Turkey said earlier on Tuesday that it opposes the involvement of the PYD or its armed wing, the People's Protection Units (YPG), in the Geneva talks.
Turkey's Foreign Minister, Mevlut Cavusoglu, said that Turkey will boycott the talks if the PYD are invited.
The opposition's High Negotiations Committee (HNC), which was formed in Riyadh, said it would decide on Wednesday whether to take part.
The committee said it is waiting for a response from the UN to a set of requestsand if their conditions are not met, the group will not attend the talks.
The Syrian government has already said it will attend.
Syrian army captures key town
The latest diplomatic developments came as the Syrian army announced a major victory, capturing the key southern town of Sheikh Maskin from rebel forces after weeks of fierce fighting .
The fall of Sheikh Maskin on Monday means that government forces will strengthen their hold on Deraa province, while cutting off rebel factions from key supply lines.
Deraa, the scene of the earliest protests against the Syrian government in 2011, contains routes crucial to both the Syrian army and rebel fighters.
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Multiple bombings also targeted a government-run security checkpoint in the central Syrian city of Homs on Tuesday, killing at least 20 people, Syria's state-run SANA news agency and opposition activists reported.
The attack was claimed by the Islamic State of Iraq and the Levant (ISIL) group.
The UK-based Syrian Observatory for Human Rights, which relies on a network of activists across Syria, said the death toll had climbed to 25, with 15 of the casualties being security personnel.
The Syrian conflict has killed at least 250,000 people, according to the UN, and more than half of Syria's pre-war population of 22.4 million has been internally displaced or have fled abroad.
Source: Al Jazeera