Members of the so-called High Negotiations Committee (HNC) met for the third day on Thursday in the Saudi capital, Riyadh, to discuss the issue.

HNC coordinator, Riad Hijab, said, "Tomorrow we won't be in Geneva. We could go there but we will not enter the negotiating room if our demands aren't met."

The group claims it would attend the United Nations-backed negotiations in the Swiss city after an agreement is reached on aid access to besieged towns.

Some 400,000 people are under siege in Syria as the foreign-backed militancy is wreaking havoc across much of the country.

Saudi Arabia and the opposition group have insisted that it should act as the only party representing the anti-government side.

In the same context, the opposition group has asked for "clarifications" after the UN issued invitations to other opposition figures.

The Syrian government’s delegation, headed by the country’s envoy to the UN, Bashar al-Ja’afari, will arrive as planned in Geneva for the talks on Friday.

The conflict in the Arab country has killed more than 260,000 people and forced millions from their homes since its onset in 2011.

Meanwhile, the UN special envoy for Syria, Staffan de Mistura, has warned that the talks "cannot fail."

"Now we need to hear your voice to everyone who is coming to this conference, and saying this conference must be an opportunity not to be missed," he said in a video message to the Syrian people.

Three rounds of UN-backed negotiations have so far been held in Vienna and New York on the situation in Syria since last October, Press TV reported.

S/SH