The Syrian government is clawing back territory from rebels with military help from Iran and Russia. It has said it is ready to attend the negotiations, which U.N. envoy Staffan de Mistura plans to hold in an indirect format.
An opposition representative said the delegation might turn up if rebels' demands were met in a day or two, but the chances of that appeared vanishingly slim.
The turn of events is a bitter blow to de Mistura, whose office had issued a video message to the Syrian people, in which he said the talks were expected to happen "in the next few days." A spokeswoman for his office, speaking before the opposition statement, said the talks would begin on Friday as scheduled.
George Sabra, a member of the opposition High Negotiations Committee (HNC), said, "For certain we will not head to Geneva and there will not be a delegation from the High Negotiations Committee tomorrow in Geneva."
Before agreeing to talks, the HNC sought U.N. guarantees of steps such as a halt to attacks on civilian areas, a release of detainees and a lifting of blockades. The measures were mentioned in a U.N. Security Council resolution approved last month that endorsed the peace process for Syria.
Sabra said a response from de Mistura was "unfortunately still ink on paper … We are not certain that the opportunity is historic," he told Arabic news channel Arabiya al-Hadath.
Another HNC official, Monzer Makhous, said the opposition parties could attend if their demands were met "within two, three or four days." He told Al-Hadath, "Tomorrow will probably be the start for those who attend, but it has no value."
The talks were meant to start in Geneva on Monday, but the United Nations has pushed them back to Friday to allow more time to resolve problems, including a dispute over which groups should be invited to negotiate with the government.
The exclusion of the PYD, a powerful Kurdish faction that controls wide areas of northern Syria, triggered a boycott by some of the invitees. Turkey, which views the PYD as a terrorist group, opposed its participation.
U.S. Secretary of State John Kerry, who is among those pushing for negotiations to start on Friday, urged the opposition to seize the "historic opportunity" and enter talks without conditions to end the war, which has displaced more than 11 million people.