Russian jets are reported to have intensified attacks on Syrian rebel positions, hours before a cessation of hostilities is due to come into force.
Russia said it was continuing to bomb "terrorists" in parts of Syria.
Earlier, almost 100 rebel factions agreed to respect the truce, the main Syrian opposition group has said.
The High Negotiations Committee (HNC) said Free Syrian Army factions and the armed opposition had signed up to the truce from midnight (22:00 GMT).
The temporary "cessation of hostilities" involves government and rebel forces - but not the so-called Islamic State (IS) group and the al Qaeda-linked Nusra Front. On Friday, Nusra Front urged its supporters to intensify attacks against President Bashar al-Assad and his allies.
Meanwhile, UN envoy to Syria Staffan de Mistura said that peace talks would resume on 7 March, if the fighting stops and aid is delivered as planned.
Warring parties in Syria were meant to make their intentions known by midday Friday ahead of the pause in fighting.
Announcing the intentions of rebel factions, the HNC said the Syrian government and its allies must not use the "proposed text to continue the hostile operations against the opposition factions under the excuse of fighting terrorism".
Analysis from Jonathan Marcus, BBC defence and diplomatic correspondent
The stage is set for the potential implementation of some kind of a ceasefire in Syria.
The Syrian government, the Russians, the US and its allies, along with just under 100 rebel groups, all appear willing to come on board.
But if it goes into effect at all, this "cessation of hostilities" - some prefer to call it a temporary truce - is going to be incomplete, partial, and fundamentally fragile.
Overnight, Russian air strikes which were "more intense than usual" hit rebel bastions including Eastern Ghouta east of Damascus, northern Homs province and western Aleppo province, the Syrian Observatory for Human Rights monitoring group said.
"It's more intense than usual," Observatory head Rami Abdel Rahman was quoted by news agency AFP as saying. "It's as if they [the Russians and the government] want to subdue rebels in these regions or score points before the ceasefire."
The Observatory said the rebel-held Damascus suburb of Douma had also suffered heavy air strikes on Friday, killing eight people, four of them children.
It said the Syrian government had also shelled the area, which is a stronghold of the Army of Islam rebel group.
But Russian President Vladimir Putin said his forces were targeting IS, Nusra Front and other extremist groups designated as legitimate targets by the UN Security Council, adding that "the decisive fight against them" would "without doubt, be continued".
Turkey said the Russian and government strikes in the run-up to the cessation gave "serious concerns".
President Recep Tayyip Erdogan's spokesman, Ibrahim Kalin, said Turkey had "played an active role" in drafting the ceasefire but "the fact that Russian planes' bombardments and Assad's forces' attacks on the ground have been continuing... gives us serious concerns about the future of the ceasefire".
The cessation has been brokered by the US and Russia but scepticism has lingered over the plan.
BBC Arab affairs editor Sebastian Usher says all sides have made it clear they will fight if attacked.
German Foreign Minister Frank-Walter Steinmeier expressed "great concern" over fresh reports that pro-government forces had dropped barrel bombs on the Damascus suburb of Darayya.
He urged all parties "to refrain from steps that could endanger the ceasefire so close to it coming into effect".
Earlier, US President Barack Obama said the success of the cessation would depend on whether warring parties including the Syrian government, Russia and their allies lived up to their commitments.
Attacks needed to end, he said, and humanitarian aid had to be allowed through to desperate civilians.
"The coming days will be critical and the world will be watching," he added.
Russian Foreign Minister Sergei Lavrov said he hoped the US would also respect the truce.
More than 250,000 Syrians have been killed in nearly five years of civil war. Millions more have been displaced.