The High Negotiating Committee (HNC) announced the refusal after an unprecedented meeting with a Russian minister, whose country is a key ally of President Bashar al-Assad.
“We will not deal with it, and if (UN mediator Staffan de Mistura) adds it in any time we will not deal with it or discuss it,” HNC delegation chief Yehya Kodmani told reporters.
The issue has overshadowed the Geneva talks, in particular after a suicide assault in Syria s third-largest city of Homs last weekend which killed dozens of people.
On Tuesday Russia called for terrorism to be added to the agenda of the talks, which have until now focussed on three “baskets”, or areas for discussion: governance, constitution and elections.
According to Syria s SANA news agency, citing sources close to the regime, UN envoy de Mistura has agreed that the talks broach four areas: the three previously agreed plus terrorism.
The veteran UN mediator, arriving at the same Geneva hotel as Kodmani a short time later, declined to comment, saying he would speak at the end of the current round of talks.
Russian deputy foreign minister Gennady Gatilov was cautious after Wednesday s meeting with the HNC delegation.
“This was a sound meeting, constructive discussion about the Syrian negotiation round. There is a common understanding that we must move toward political regulation of the Syrian crisis,” he said, quoted by the RIA-Novosti news agency.
“What is new about these Geneva talks is that the sides agreed with the suggestion of de Mistura to discuss all issues in a parallel mode, so that there are several tracks.”
He added: “Of course we would like to see direct talks between (the) Syrian government delegation and a united opposition delegation, but we must admit that this is not attainable right now.
The Syrian regime s chief negotiator Bashar al-Jaafari demanded at the weekend that all factions at the Geneva talks condemn the Homs attack or be deemed “accomplices of terrorism”.
UN Syria envoy de Mistura launched the latest round of talks last Thursday, but as in previous sessions there appears little prospect of the two sides meeting face-to-face.
The opposition says political transition — specifically the fate of President Assad — should be added to the discussions.
The Geneva talks, the fourth round of UN-sponsored talks in the six-year conflict which has killed over 310,000 people, are expected to end before or during the coming weekend, though no formal time frame has been set.
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