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.
Meanwhile, almost 100 rebel factions have 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 regime and rebel forces – but not the so-called Islamic State (IS) group and the al Qaeda-linked Nusra Front.
Warring parties in Syria were meant to make their intentions known by midday (10:00 GMT) 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”.
Overnight, Russian air strikes which were “more intense than usual” hit rebel bastions including at Eastern Ghouta east of Damascus, in the north of Homs province and in the west of 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.”
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”.
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.
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.
He said attacks needed to end 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.