MONTREUX, Switzerland- Nearly three years after the start of their fierce civil war, Syria's opposing sides gather with world powers in Switzerland Wednesday for the most serious effort yet to end the bloodshed.

Meeting for the first time since the start of the conflict, the two sides could not be further apart as the "Geneva II" conference kicks off in Montreux on the shores of Lake Geneva.

The opposition is heading into the conference with a sole aim -- toppling President Bashar al-Assad -- while the regime says any talk of removing the Syrian leader is a "red line" it will not cross.

So expectations are low, but top global diplomats gathered for the conference believe that simply bringing the two sides together is a mark of some progress and could be an important first step.

"It would be wrong to expect progress in the next few days in terms of major breakthroughs," British Foreign Secretary William Hague said on arrival Tuesday.

"Nevertheless, things can be achieved once diplomacy starts, once diplomacy is attempted -- we've seen that on many other subjects, including with Iran on its nuclear programme."

The conference was to begin with formal speeches by UN leader Ban Ki-moon, US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov.

The opposing Syrian sides are then expected to have their say, followed by representatives of the rest of the about 40 nations and international groups invited to Montreux.

No direct talks are expected until Friday, when opposition and regime delegations will meet in Geneva for negotiations that officials have said could last seven to 10 days.

Erupting after the regime cracked down on protests inspired by the Arab Spring, the civil war has claimed more than 130,000 lives and forced millions from the homes.

Recent months have seen the conflict settle into a brutal stalemate -- with the death toll rising but neither side making decisive gains.

With no one ready for serious concessions, world powers will be looking for short-term deals to keep the process moving forward, including on localised ceasefires, freer humanitarian access and prisoner exchanges.

Both Washington, which supports the opposition and has called for Assad to step down, and Moscow, the regime's key international supporter and arms provider, will be pushing for progress but officials have warned the talks will be extremely difficult.

"Everybody has to understand that this is the beginning of a process. It's not going to be fast. It's very bitter fighting on the ground. And so there's going to be an absolute requirement for patience and for persistence," a senior US State Department official said.

Notably absent from the talks will be crucial Assad backer Iran, after Ban reversed a last-minute invitation when the opposition said it would boycott if Tehran took part.

It took months of discussions to convince all sides to participate, with the main opposition group, the Syrian National Coalition, only agreeing at the 11th hour.

That move has seen the Coalition branded as traitors by some in Syria, including Islamist rebels who have often been at the forefront of fighting.

Pitting Assad's regime, dominated by the Alawite offshoot of Shiite Islam, against largely Sunni Muslim rebels, the conflict has unsettled large parts of the Middle East.

Shiite Iran and its Lebanese militia ally Hezbollah have backed Assad; the mainly Sunni Arab Gulf states have supported the opposition; and the violence has often spilled over into neighbouring Lebanon and Iraq.

There were stark reminders of the conflict's impact in the run-up to the talks, with continued fighting on the ground and new evidence alleging that Assad's forces have systematically killed and tortured 11,000 people.

Three former international prosecutors said a defector had provided the evidence of starvation, strangulations and beatings, as well as pictures showing emaciated corpses with grisly wounds.

One of its authors -- the former chief prosecutor of the special court for Sierra Leone, Desmond de Silva -- said it was the "smoking gun" proving "industrial-scale" killing by the Syrian regime.