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 UN-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, UN 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.
A prominent Syrian dissident who is not part of the Saudi-backed HNC, Haytham Manna, said he would stay away from the talks, which he regarded as a "failing project".
Manna, co-leader of the Syrian Democratic Council that includes Kurdish members, boycotted the last round of talks because the Kurds were not included.
Russian Foreign Minister Sergei Lavrov said however that de Mistura should this time include representatives of Kurdish groups, which have been fighting in Syria.
Kurdish groups such as the PYD party and its affiliated YPG militia have not been invited so far. Regional power Turkey does not want them in Geneva and views the YPG as a terrorist group. Russia says the Kurds are a legitimate part of a future Syria, and should be at the table.
There has been speculation that they will be included in the coming round. De Mistura says he has not expanded the list of invitees, but the talks' format gives him flexibility to consult whomever he wants.
PYD co-chair Saleh Muslim said Kurds should be included for any political settlement to work.
The cessation of hostilities agreement which came into force on Feb. 27 does not include the two main jihadist groups, Islamic State and the Nusra Front.
A source close to the government said the Syrian army, backed by Russian air strikes, is aiming to capture the historic city of Palmyra from Islamic State and open a road to the eastern province of Deir al-Zor, where the jihadists are also established.
The Russian air force has hit Palmyra with dozens of air strikes since Wednesday, according to the Syrian Observatory for Human Rights monitoring group.
Syrian government forces were on Friday battling Islamic State 7 km (4 miles) from the ancient site that fell to the jihadists last May.
Islamic State has blown up ancient temples and tombs since capturing Palmyra in what the UN cultural agency UNESCO has called a war crime.
Islamic State attacks
The capture of Palmyra and further eastward advances into Deir al-Zor would mark the most significant Syrian government gain against Islamic State since the start of the Russian intervention last September.
The momentum has turned against Islamic State since its rapid advances two years ago following the capture of the Iraqi city of Mosul. Its finances are also under strain.
The group's tactics in Syria appear to reflect the strains, as it turns to suicide missions seemingly aimed at causing maximum casualties rather than sustainable territorial gains.
The group said on Friday two of its members carried out suicide attacks against Kurdish fighters in at the Tishrin dam in Aleppo province, which Kurds and Arab rebel groups captured in December.
Warplanes also hit areas of western Syria on Friday, the Observatory said. An air raid by the government side killed at least five people in a rebel-held area of Aleppo.
It also reported clashes between insurgents and government forces in the northern Latakia countryside. SANA said the army had taken over several villages in the area. There was no independent confirmation of the gains.
In northern Aleppo province clashes continued between Kurdish fighters and insurgents, in a another separate fight which has pitted the YPG and its allies against rebels supported through Turkey.