Powerful armed groups in Syria said attending peace talks or negotiating with the regime would be an act of betrayal, as fighting raged on Sunday near Iraq and in Homs.
An international watchdog said, meanwhile, that Syrian President Bashar al-Assad's regime handed over on time a detailed plan on destroying its chemical weapons stockpile.
The move was in line with a US-Russian deal reached last month that headed off threatened military strikes on Syria and that triggered an initiative for peace talks to be staged in Geneva next month.
But the initiative has struggled to win the support of the warring parties in Syria, where more than 115,000 people are estimated to have been killed in the 31-month conflict.
In the latest blow, 19 Islamist groups fighting to topple Assad issued a statement casting further doubt on whether the talks dubbed "Geneva 2" will actually go ahead.
"We announce that the Geneva 2 conference is not, nor will it ever be our people's choice or our revolution's demand," said the statement read out by Suqur al-Sham brigade chief Ahmad Eissa al-Sheikh in a video posted online.
"We consider it just another part of the conspiracy to throw our revolution off track and to abort it."
They warned anyone who attends such talks would be committing "treason, and... would have to answer for it before our courts".
The statement comes as the UN-Arab League peace envoy to Syria, Lakhdar Brahimi, prepares to travel to Damascus on Monday, a Syrian government source told AFP.
The talks slated for next month aim to bring rebel and regime representatives to the table in a bid to seek a negotiated end to the conflict.
Under pressure from its Western backers to attend, the National Coalition is to meet on November 9 to decide whether to take part.
But it insists it will only do so if there are guarantees Assad will step down, and its leader Ahmad Jarba has also said no talks can take place unless the regime frees women and children from its jails.
Assad for his part has said "the factors are not yet in place" for such talks, and he has repeatedly rejected negotiations with any group with ties to the rebels or to foreign states.
In a separate development, Damascus handed over a detailed plan to destroy its chemical stockpile on time, the Organisation for the Prohibition of Chemical Weapons said.
"On 24 October 2013, the Syrian Arab Republic submitted to the OPCW its formal initial declaration covering its chemical weapons programme," it said, adding Damascus had had until Sunday to do so.
Damascus was required to submit the plan under a UN Security Council resolution that also called for a political settlement to end the war.
On Syria's eastern frontier with Iraq, Kurdish militia took over most of the town of Yaarubiyeh a day after they routed jihadists from the border post, said the Syrian Observatory for Human Rights.
By Sunday, the Kurds had "near-total control" over the town, said the group, adding that fighting was still raging in the south of Yaarubiyeh.
The Islamic State of Iraq and the Levant, an Al-Qaeda front group fighting the Kurds there, called on jihadists to join them in the fight over Yaarubiyeh, said the Observatory.
The opposition Coalition said Saturday that Iraq "continues to interfere with Syria's internal affairs", and that its troops "shelled" the Yaarubiyeh border post in cooperation with the Kurdish fighters.
The Observatory said the allegations were unfounded.
In the central province of Homs, battles pitting rebels against regime troops as opposition fighters pushed an advance to take over major weapons depots in the area.
One of the battlegrounds is Sadad, a Christian town in the province where shelling killed two men and three women from the same family.
The Assyrian Human Rights Network appealed Saturday to fighters on both sides "for an immediate ceasefire" in Sadad.
Such a ceasefire "is needed, even if just for a few hours, to evacuate the killed and wounded civilians, and to bring in medicines, water and other humanitarian supplies to the civilians trapped in their homes since last Monday".