Members of the so-called High Negotiations Committee - formed in Riyadh last month in an effort to unite Syria's fractious opposition - met in the Saudi capital for a second day on whether to accept a UN invitation to the talks due to start Friday.
The Committee insists it must be the sole opposition delegation at the talks in Geneva and is asking for "clarifications" after the UN issued invitations to other regime opponents.
Committee spokesman Monzer Makhous told AFP the talks had resumed and could last all day but refused to comment further.
Diplomats, including US Secretary of State John Kerry who met with Committee members at the weekend, have piled pressure on the opposition to attend.
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The talks are part of a UN-backed plan agreed by top diplomats last year in Vienna that envisages negotiations, followed by the creation of a transitional government, a new constitution and elections within 18 months.
UN Syria envoy Staffan de Mistura's office yesterday said it had issued invitations to the talks, but refused to say who had been invited.
The Committee, which earlier this month named Mohammed Alloush of the Islamist rebel group Army of Islam as its chief negotiator, confirmed it had received an invitation but so did several other opposition figures who do not belong to the body.
It was not clear whether the other figures had been invited as official delegates or simply observers to the talks.
The row over who will attend the talks reflects not only internal divisions but also the interests of the various diplomatic powers embroiled in the Syria conflict.
Russia, a key ally of Assad, has pushed for a broader range of opposition at the talks, with Foreign Minister Sergei Lavrov saying yesterday that negotiations "will not achieve results" if Syria's Kurds are not represented.