Syria's opposition leader has laid out his demands for talks on ending the country's civil war, calling for the creation of a transitional government that does not include President Bashar al-Assad.

Ahmed Jarba told reporters in Geneva that he was looking to a future without Mr Assad.

In a defiant speech, Mr Jarba said the international community had now realized that Mr Assad cannot stay in power.

"We have started to look into the future without him. Assad and all of his regime is in the past now. Nobody should have any doubt that the head of the regime is finished. This regime is dead," Mr Jarba said.

Russia, which is Mr Assad's major backer, had given assurances that it was not "holding on" to Mr Assad, Mr Jarba said.

Mr Assad's officials, who left talks with a UN envoy without making any statement, have insisted that the Syrian president is not going anywhere, a view endorsed by US Secretary of State John Kerry.

"Obviously he is not ready at this point in time," Mr Kerry said in an interview with Al-Arabiya television.

The opposing views of Mr Assad's future underlined the difficulties facing the Geneva talks, the first time the rival sides have met since the fighting began.

Mr Jarba said the talks would be difficult, long and would look at all "core issues" as a package deal, including the creation of a transitional governing body.

"This is the basis of our negotiations and we will demand it," he said.

The first day of talks was dominated by fierce rhetoric from Mr Assad's government and its foes, with each accusing the other of atrocities and showing no sign of compromise.

Few expect the peace talks to result in a breakthrough to end the war, but officials hope they can salvage the process by starting with more modest, practical measures to ease the plight of millions of people on the ground, especially in areas cut off from international aid.

Islamist fighters who disdain the Western and Arab-backed opposition are not present at the talks, and nor is Iran, Mr Assad's main regional backer.

More than 130,000 people are believed to have been killed, nearly a third of Syria's 22 million people have been driven from their homes and half are in need of international aid, including hundreds of thousands in areas cut off by fighting.

Yesterday's opening ceremony saw global powers vigorously defend their sides, with Western countries, Arab states and Turkey all joining the opposition in demanding a transitional government that would exclude Mr Assad.

Russia, his main global supporter, said the focus of talks should be on fighting "terrorism", a word the Syrian government applies to all of its armed opponents.

The main negotiations, expected to last up to a week, are due to begin on Friday at the United Nations headquarters in Geneva.

The two sides are not expected to meet face-to-face for now, leaving UN officials to act as go-betweens.

One of the opposition negotiators has spoken of a two-stage process, with practical steps such as prisoner swaps, ceasefires, the withdrawal of heavy weapons and setting up aid corridors being dealt with first, before the political future.

The talks remain fragile, however, with both sides threatening to pull out.

The government said it will not discuss removing Mr Assad, while the opposition said it will not stay unless Mr Assad's removal is the basis for talks.

Among the many difficulties with the process, the opposition delegation does not include the al Qaeda-linked Sunni Islamist militant groups who control much of the territory in rebel hands and have denounced those attending the talks as traitors.

Rebel ranks have been divided, with hundreds killed in recent weeks in battles between rival factions and the al Qaeda-linked Islamic State of Iraq and the Levant.

Al Qaeda leader Ayman al-Zawahri has called on fighters to unite.