Syria conflict: Government sets conditions for new peace talks
The Syrian government says it will send a delegation to fresh peace talks due to start in Geneva on Monday, but has put limits on the agenda.
Foreign Minister Walid Muallem, speaking in Damascus, ruled out any debate of presidential elections.
An opposition spokesman said the government's pre-conditions could halt the talks before they had even started.
US Secretary of State John Kerry is expected to meet his German, French, and British counterparts on Sunday.
Earlier, the US said violence had fallen dramatically since a cessation of hostilities began two weeks ago.
The main Syrian opposition umbrella group, the High Negotiations Committee (HNC), said on Friday it would push for an interim government with full executive powers in which President Bashar al-Assad and the current leadership would have no role.
The fate of President Assad has been one of the main stumbling blocks in previous rounds of tentative talks.
UN special envoy to Syria Staffan de Mistura has said he wants to see presidential elections in the next 18 months, but on Saturday Mr Muallem dismissed the idea.
"Neither he nor anyone else has the right to talk about presidential elections. This is an exclusive right of the Syrian people,'' he said.
He described the opposition's demands for an interim government with full executive powers as a "red line".
"If this is what they [the opposition] think, I advise them not to come to the negotiations so that they don't make us tired and we don't make them tired," Mr Muallem said.
He said the Syrian government delegation would return to Damascus within 24 hours if the opposition did not turn up.
HNC spokesman Monzer Makhous accused Mr Muallem of "putting the nails in the coffin of Geneva".
"Muallem is stopping Geneva before it starts," he told Al Arabiya Al Hadath TV.
A fragile process - Lyse Doucet, BBC chief international correspondent
Syria's warring sides are under significant pressure from their respective backers to go to Geneva. No one wants to be blamed for any breakdown. So they are likely to keep their pledge to show up.
But there is an unwavering red line for Damascus - no discussion of President Assad's role, not even of presidential elections. Never mind that UN envoy Staffan de Mistura said elections would be discussed. Mr Muallem - and presidential adviser Bouthaina Shaaban in a BBC interview - say Mr de Mistura could not "dictate" the agenda.
The Syrian government has been emboldened by Russia's immense military support, as well its diplomatic weight which is shaping this new process. It is still not clear what Russia expects in Geneva. And the opposition, and its backers, have their own red line on President Assad's future.
Despite a truce that, unexpectedly, has largely held for two weeks, the gaps remain dangerously wide in these very early stages of a very fragile process.
The HNC has already said it will attend the UN-sponsored indirect talks in Geneva. The last round collapsed in February without agreement.
A temporary cessation of hostilities agreed by most participants in the conflict began at the end of last month. It excludes so-called Islamic State (IS) and al-Nusra Front, al-Qaeda's branch in Syria.
US Secretary of State John Kerry said on Saturday that US and Russian officials were meeting to discuss alleged breaches of the temporary truce by government forces.
But he said "perceived" violations of the cessation of hostilities should not derail the Geneva talks.
"The level of violence by all accounts has been reduced by 80-90%, which is very, very significant," he said.
"We believe that the start of talks this next week in Geneva presents a critical moment for bringing the political solution to the table that we've all been waiting for, even as we extend the humanitarian assistance and continue to try to reduce the violence."
The Syrian Observatory for Human Rights, a monitoring group, said air raids by the Syrian government had killed seven civilians in Aleppo on Friday. The claim cannot be independently verified.
More than 250,000 Syrians have been killed and millions more have been forced from their homes in five years of Syria's civil war that began with an uprising against Mr Assad.