Syrian government negotiators indicated Friday they might not return to continue peace talks in Geneva after a weekend break, condemning "provocative" statements from the opposition about the fate of President Bashar al-Assad.

The regime's chief negotiator Bashar al-Jaafari told reporters in Geneva that his team would go home to Syria on Saturday and that "Damascus will decide" if they will return to the UN-backed talks on Tuesday.

His comments, slammed by the opposition as irresponsible, came a day after United Nations special envoy Staffan de Mistura voiced optimism about the round of peace talks that began on Tuesday and said he would extend the negotiations for an additional two weeks.

De Mistura told reporters the parties would take a break over the weekend and resume negotiations Tuesday, with the talks expected to last until December 15.

But after meeting with de Mistura Friday, Jaafari said there were "big problems in this round of talks," pointing to a communique published by the opposition last month in Riyadh signalling it was maintaining its hard line on Assad's removal.

The opposition, united in one delegation for the first time, has defied calls to give up on its demand that Assad must step down before any peace deal can be reached.

- 'Provocative, irresponsible' -

"The language was provocative, irresponsible politically speaking," Jaafari said, warning that if the opposition delegation sticks to such language "there will be no progress."

"Those who want to impose preconditions are unrealistic," Jaafari said, pointing to shifting conditions on the ground where the government with Russian military support has made major advances.

"There is a political reality on the ground. We are the strong party," he said.

The government delegation had initially refused to confirm it would attend the talks and then arrived a day late, reportedly after securing key concessions, including keeping Assad's fate off the table.

De Mistura himself urged the opposition in September to be "realistic enough to realise they didn't win the war", and has urged both sides to approach the talks with "no preconditions."

Speaking to reporters before a brief meeting with de Mistura's deputy Ramzy Ezzeldin Ramzy Friday afternoon, opposition spokesman Yehia al-Aridi meanwhile said his delegations would be coming back next week.

He slammed Jaafari's suggestion that the government delegation might not continue participating in the process.

"Not coming back is a pre-condition in itself," he said, accusing Damascus of exhibiting "irresponsibility towards people who have been suffering for seven years now."

With little achieved through seven previous rounds of talks, pressure is mounting on de Mistura to show results.

Amid the uncertainty over the Geneva talks, Moscow has proposed holding a "congress" to bring together the government and opposition in Sochi, the Russian Black Sea resort but has not set a date.

De Mistura has strenuously insisted that the talks he is leading are the only ones that can lead to a political solution to the crisis.

On Friday evening, he published a document suggesting 12 principles for a future Syria he believed the two sides could agree, including that the country "shall be democratic and non-sectarian".

He asked the parties to discuss the points and add their thoughts for next week's talks, which should focus on drafting a new constitution and elections.

On the ground in Syria, the war is raging in some places.

On Friday, rebels shot down a Syrian regime helicopter near the Lebanese border, killing three officers onboard, according to Britain-based Syrian Observatory for Human Rights monitor.

Elsewhere though the fighting seems to have subsided.

Residents of Kurdish-dominated areas in northern Syria voted Friday in local polls aimed at boosting their region's de facto semi-autonomous status acquired on the back of the country's ongoing war.

- China offers reconstruction help -

Syria's conflict has since March 2011 claimed more than 340,000 lives, forced millions to flee their homes and left Syria in ruins.

The UN has said it will cost at least $250 billion to rebuild the country.

China's special envoy to the Syrian conflict, Xie Xiaoyan, told reporters in Geneva Friday that Beijing was "ready to participate in the reconstruction phase when it starts."

He said the "Chinese government would encourage Chinese firms to undertake projects" in the country once the security situation there is under control.