The UN-brokered indirect negotiations due to begin in Geneva on Monday are the latest international push to find a solution to Syria's five-year civil war which has killed more than 270000 people.

Both the government and the main opposition group the High Negotiations Committee (HNC) have agreed to attend after the last talks collapsed in February.

But the fresh push for peace was already facing questions on Saturday as the two sides wrangled over Assad's fate.

"We will not talk with anyone who wants to discuss the presidency... Bashar al-Assad is a red line" Syrian Foreign Minister Walid Muallem told a Damascus news conference.

"If they continue with this approach there's no reason for them to come to Geneva" he said.

The HNC has repeatedly called for Assad's departure as a prerequisite for any deal and chief opposition negotiator Mohammad Alloush said the president must go for peace talks to stand a chance.

"We believe that the transitional period should start with the fall or death of Bashar al-Assad" he told AFP in a joint interview in Geneva.

"It cannot start with the presence of the regime or the head of this regime still in power."

UN peace envoy Staffan de Mistura has said the Geneva meetings would not last more than 10 days.

The negotiations are set to cover the formation of a new government a fresh constitution and UN-monitored presidential and parliamentary elections within 18 months.

- 'Future without Assad' -

Assad's fate has been one of the main stumbling blocks in previous rounds of tentative talks with key ally Russia rejecting any suggestion he should go while opposition-backer the US has called for him to step down.

Saudi Arabia's Foreign Minister Adel al-Jubeir said Saturday: "I would emphasise that people see the future of Syria without Bashar al-Assad."

But Muallem said it was not De Mistura's place to raise the question of presidential elections.

"Neither he nor anyone else whoever they may be has the right to discuss presidential elections" Muallem insisted. "This right is exclusively for the Syrian people."

Muallem said the negotiations would aim to form a "unity government" which would then appoint a committee to either write a new constitution or amend the existing one.

"Then we will have a referendum for the Syrian people to decide on it" he said.

The HNC has called for the creation of a transitional body with full executive powers and Alloush said Muallem's comments "show that the regime is not serious about the political process".

There have also been questions about how far any deal would be felt on Syria's battlefields where myriad groups have been competing for territory.

Russia had called on De Mistura to include Syrian Kurds in peace talks but the envoy told Swiss newspaper Le Temps that while they would not take part they should be given a chance to express their views.

- 'Significant' fall in violence -

Fighting has eased across Syria since a landmark ceasefire between Syria's regime and rebels bar some Islamist groups took effect two weeks ago.

US Secretary of State John Kerry said the truce had reduced violence by 80-90 percent since coming into force on February 27 which he described as "very very significant" news ahead of the talks.

He said US and Russian officials would meet in Geneva and Amman on Saturday to discuss opposition complaints of violations.

"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" he said after meeting top officials in Saudi Arabia.

Both sides have accused the other of breaking the truce and Alloush said there have been 350 violations since it began which showed the regime was "not serious" about the ceasefire.

Regime air raids killed seven civilians in rebel-held areas of second city Aleppo on Friday the Syrian Observatory for Human Rights said.

The Britain-based monitor said an Islamist rebel group claimed to have shot down a regime warplane Saturday in central Hama province but a pro-government Facebook page blamed "technical difficulties".

Elsewhere at least three rockets fired from Syria hit the Turkish border town of Kilis without causing casualties the local governor's office said.