A Syrian opposition committee has named an Islamist rebel as its chief negotiator at peace talks that the UN hopes to convene in Geneva on Monday.
Mohammed Alloush is the political leader of the powerful, Saudi-backed group Jaysh al-Islam (Army of Islam).
Both the Syrian government and its staunch ally, Russia, consider Jaysh al-Islam a terrorist organisation.
The opposition committee also warned that it would pull out of the talks if a third party was invited to attend.
Russia wants opposition groups tolerated by President Bashar al-Assad to participate in the negotiations on a political solution to the conflict in Syria, which has left more than 250,000 people dead since 2011.
Kurdish groups, which control large parts of the north, also want to attend.
'Dates are not sacred'
An unprecedented meeting of Syrian opposition politicians and rebels in Riyadh last month led to the creation of a committee to oversee the talks with the government.
On Wednesday, the head of the Supreme Commission for Negotiations, Riad Hijab, announced Mr Alloush's appointment as its chief negotiator.
Jaysh al-Islam controls much of the eastern outskirts of the capital Damascus. Its leader, Zahran Alloush, was killed in an air strike last month.
Russia says Jaysh al-Islam differs little from the jihadist group Islamic State (IS), but the two are violently opposed to each other and Zahran Alloush said before his death that he favoured allowing Syrians to decide whether they wanted Islamic rule.
Asaad al-Zoubi, a former Syrian army general, will serve under Mr Alloush as head of the opposition negotiating team, while his deputy will be George Sabra of the Syrian National Council.
Mr Hijab warned that the committee would not accept any attempts by foreign parties to "inject individuals in the form of a so-called third delegation, justifying their presence under unfounded pretexts merely to disrupt the political process and prolong the fighting in the name of combating terrorism".
The former prime minister also said that the opposition could not negotiate while Syrians "suffer from shelling, starvation and siege" by government forces.
"Dates are not sacred," he added. "Debased political bartering at the expense of the Syrian people is tantamount to callous extortion which we will not accept under any circumstance."
The UN has said it will not issue invitations to the peace talks until major powers backing the government and opposition had agreed on who should attend.
After a meeting with US Secretary of State John Kerry in Switzerland on Wednesday, Russian Foreign Minister Sergei Lavrov told reporters they were not considering postponing the talks.
"We hope the negotiating process will begin this month," he said. "I stress that this will be just the start, because of course it will take a lot of time, a whole range of arduous tasks are to be resolved."