Only Syrians have been invited to peace talks in Geneva, the UN said Wednesday, in an apparent contradiction to Turkey's suggestion that it would be included.

On Tuesday, Turkish Foreign Minister Mevlut Cavusoglu said Ankara would "boycott" the talks set to open on Friday if the Syrian Kurdish group Democratic Union Party (PYD), which Ankara believes is linked to militants fighting inside Turkey, was at the negotiating table.

But Khawla Mattar, a spokeswoman for the UN's Syria envoy Staffan de Mistura said there was "no plan to invite" non-Syrians when asked about the possible inclusion of observer delegations from Turkey, Russia, the United States or France.

She declined to comment on which parties had been given invitations, which were issued by de Mistura on Tuesday.

Moscow has said the PYD's inclusion was key to the talks' success.

Turkey's objection threatens to be a major blow to the negotiations, which have already been delayed by a stalemate over the make-up of the opposition delegation.

Members of the Syrian opposition's so-called High Negotiations Committee (HNC) were meeting in Riyadh on Wednesday to decide whether to attend the UN-brokered talks.

The HNC has asked de Mistura to send the full list of proposed participants before making its decision.

Asked whether the UN envoy had complied, Mattar said there were "no pre-conditions" attached to joining the talks, suggesting the HNC would not be able to view the guest list before deciding whether to come to Geneva.