Syria's main opposition group said it would attend peace talks on Monday but accused the government of President Bashar al-Assad of preparing to escalate the war to strengthen its negotiating position.

The U.N.-brokered talks, which coincide with the fifth anniversary of the conflict, will take place in Geneva two weeks after the start of a ceasefire agreement.

The truce deal has reduced violence although not halted the fighting, with further hostilities reported in western Syria on Friday, and as battles against Islamic State raged further east.

The High Negotiations Committee said on Friday it would attend the peace talks as part of its "commitment to international efforts to stop the spilling of Syrian blood and find a political solution".

But it played down any chance of reaching agreement with the Syrian government to end the war that has killed more than 250,000 people and led to a refugee crisis in the Middle East and Europe.

Russia said it expected its ally Syria to attend, although Damascus has yet to publicly confirm it will do so. The Syrian foreign minister is expected to announce his government's position on the talks on Saturday.

Peace talks convened two years ago collapsed because the sides were unable to agree an agenda: Damascus wanted a focus on fighting terrorism, the term it uses for the rebellion, while the opposition wanted to discuss a transitional government.

A senior adviser to Assad, Bouthaina Shaaban, said on Friday Russia had done more to fight terrorism in Syria than the United States and its allies, according to a BBC interview cited by state news agency SANA.

The latest talks are intended to focus on future political arrangements in Syria, a new constitution and elections, U.N. envoy Staffan de Mistura said.

The opposition HNC said it wanted the talks to concentrate on the establishment of an interim governing body with full executive powers.

HNC coordinator Riad Hijab said the group was "concerned with representing the just cause of the Syrian people ... and investing in all available chances to alleviate the Syrian people's suffering".

"We know that they (the government) are committing crimes, and that they are preparing an air and ground escalation in the coming period," he said, without elaborating.

HNC spokesman Salim al-Muslat said they expected a government escalation with the aim of strengthening Damascus's position at the negotiating table. "I believe this is a strategy," he said. – Reuters