Syria opposition officials Wednesday criticised UN envoy Staffan de Mistura after he said he could pick the opposition delegation representatives to UN-led talks in Geneva if they failed to do so.

De Mistura on Tuesday said the talks previously scheduled for February 8 had been postponed to February 20 in part to give the opposition more time to present a united delegation.

And he warned that if they had not done so by February 8, he would "select the delegation in order to make sure that it can be as inclusive as possible".

The opposition rejected the comments as "unacceptable."

"Selecting the Syrian opposition delegation is not the business... of de Mistura," wrote Riad Hijab, head of the opposition High Negotiations Committee, on his Twitter account.

Salem Muslet, a spokesman for the opposition umbrella group, said de Mistura's comments were "unacceptable" and showed a "disregard for the ability of the representatives of the Syrian people".

De Mistura announced that the UN-led talks were being delayed to "give a chance both to the government to become seriously engaged in discussions and the opposition... to actually be able to be given a chance to come with one unified opposition".

But Muslet rejected the suggestion that the opposition was responsible for the delay, which he said was "not in the interests of the Syrian people".

The delay was "a response to the demands of the regime's allies", he argued.

He also asked whether de Mistura would "intervene in the formation of (President Bashar al-)Assad's delegation".

The Geneva talks will be the first since a round of negotiations sponsored by Assad allies Russia and Iran, and rebel backer Turkey, held in the Kazakh capital Astana last month.

Those talks ended without any major breakthrough, though a nationwide Syria truce brokered by Russia and Turkey has largely held since it began on December 30.

More than 310,000 people have been killed in Syria since the conflict erupted with anti-government protests in March 2011.