The UN envoy for Syria declared Monday the official start of peace negotiations in Switzerland aimed at charting a way out of a brutal and tangled civil war that has killed more than 260,000 people.

Speaking after his first formal meeting in Geneva with the main opposition grouping, Staffan de Mistura said that indirect talks between the government and the opposition are now "starting officially".

The Swedish-Italian diplomat said he expected the talks to be "complicated and difficult," but that Syria's people deserved to "see something concrete, apart from a long, painful negotiation."

He said he could not say how long he expected a first round of talks to last, but said he hoped the negotiations would "achieve something" by February 11.

The main opposition High Negotiations Committee (HNC), here since late Saturday, had been reluctant about committing to a hoped-for six months of "proximity talks", using go-betweens to negotiate with the government of President Bashar al-Assad.

The group had demanded that before starting talks humanitarian aid gets through to besieged cities, that Russian and regime bombardment of civilians stop and that prisoners are released.

HNC spokesman Salem al-Meslet sounded upbeat after the meeting with de Mistura, vowing that his group "will strive to join the political process."

"We came here to discuss with the special envoy UN resolution 2254, lifting the sieges and stopping the crimes done by Russian air strikes in Syria, and I believe we received positive messages," he said.

- Millions flee -

The highly complex, almost five-year-old Syrian war has forced half of Syria's population to flee their homes, with millions fleeing to neighbouring states or even risking their lives reaching Europe.

The war has sucked in, on opposing sides, not only other countries in the region like Turkey and the Gulf states but also Western nations and, since September, Russia.

The chaos has allowed the Islamic State extremist group -- which has claimed responsibility for atrocities around in the world including in Paris in November -- to overrun swathes of Syria and also Iraq.

On Sunday the extremist Sunni group said it was a behind multiple bombings at a revered Shiite shrine south of Damascus that monitors said killed more than 70 people.

In November, world powers agreed in Vienna on an ambitious roadmap that foresees six months of talks leading to a new constitution and free elections within 18 months.

But they did not address the controversial question about the future of Iran and Russia's ally Assad, whose forces in recent months have made progress on the ground thanks to Moscow's military involvement.

British Foreign Secretary Philip Hammond said Monday the talks "must deliver a political transition away from Assad".

But experts say the West is backing off from demands that Assad leave before any such transition starts, seeing him as a lesser evil than IS.

- 'Not serious' -

The HNC said it was waiting to hear more from de Mistura after he meets with the delegation representing Assad's government on Tuesday.

On Sunday the government's chief negotiator, Bashar al-Jaafari, denounced the HNC as "not serious".

"We do not know who is the other side. They don't even have a final list (of negotiators)," said Syria's tall and stern ambassador to the UN.

In particular, the government objects to the inclusion in the HNC -- backed by Washington and in particular by Iran's arch rival Saudi Arabia -- of rebels who it sees as "terrorists".

One of these, the HNC's chief negotiator, is Mohammed Alloush from the powerful Islamist armed rebel group Army of Islam, or Jaish al-Islam, who arrived in Geneva late Monday.

"We came to find a solution," Alloush said after arriving, adding however: "There is no common ground with the regime. The regime wants to eliminate the opposition."

He did not confirm whether he would be meeting with de Mistura in the coming days.

In a potentially hopeful sign, Western diplomats said Sunday that Riad Hijab, the head of the HNC and a former Syrian prime minister, was also due in Geneva soon.

Outside powers were also in Geneva keeping a close eye on proceedings, with Jaafari reportedly meeting with the Russian ambassador, and Western envoys touching base with the opposition.

A US official said that Anne Patterson, US assistant secretary for Near Eastern Affairs and US Syria envoy Michael Ratney met with Russian Deputy Foreign Minister Gennady Gatilov on Monday in Geneva.

Patterson "urged Russia to use its influence with the Assad regime to push for full humanitarian access to all Syrians in need," the official said.

burs/stu/kjl