Intense Russian air strikes battered rebel bastions across Syria on Friday, a monitor said, just hours before a midnight deadline for a landmark ceasefire in the country's five-year civil war.
With the ceasefire due to take effect at 2200 GMT, US President Barack Obama has warned Damascus and key ally Moscow that the "world will be watching".
Both President Bashar al-Assad's regime and the main opposition body have agreed to the deal -- which allows fighting to continue against the Islamic State group and other jihadists.
The agreement brokered by Russia and the United States marks the biggest diplomatic push yet to help end Syria's violence, but has been plagued by doubts after the failure of previous peace efforts.
Members of the 17-nation group backing the process were meeting in Geneva on Friday to work out further details of the so-called "cessation of hostilities", which was then expected to be endorsed by the UN Security Council, diplomats said.
The Syrian Observatory for Human Rights, a Britain-based monitor, said Russia and the regime had launched a wave of attacks on non-jihadist rebel areas ahead of the deadline.
"It's more intense than usual. It's as if they want to subdue rebels in these regions or score points before the ceasefire," Observatory head Rami Abdel Rahman said.
Russia launched air strikes in Syria last September saying it was targeting "terrorists" but critics have accused Moscow of hitting rebel forces in support of Assad, a longtime ally.
The Observatory said there had been Russian strikes on rebel bastions including the Eastern Ghouta region outside Damascus, the north of Homs province and the west of Aleppo province.
There were at least 26 air strikes on Eastern Ghouta including 10 on its main city of Douma which was facing heavy regime shelling, it said.
One Douma resident told AFP that "the bombing is very heavy" while another described "very big explosions" in the city.
Russian President Vladimir Putin insisted Moscow would continue targeting "terrorist groups".
"The decisive fight against them will, without doubt, be continued," Putin said in televised remarks.
"We understand fully and take into account that this will be a complicated, and maybe even contradictory process of reconciliation, but there is no other way," Putin said.
The intensified attacks prompted Turkey, a key supporter of opposition forces, to express worries over the viability of the ceasefire.
"We are seriously concerned over the future of the ceasefire because of the continuing Russian air raids and ground attacks by forces of Assad," presidential spokesman Ibrahim Kalin told reporters in Ankara.
The complexity of Syria's battlefields -- where moderate and Islamist rebel forces often fight alongside jihadist groups like the Al-Qaeda affiliated Al-Nusra Front -- has raised serious doubts about the feasibility of a ceasefire.
Al-Nusra's chief Mohammad al-Jolani on Friday urged Assad's opponents to reject the ceasefire and instead intensify attacks on the regime.
"Beware of this trick from the West and America," he said in an audio message. "Negotiations are the ones conducted on the battlefield."
Diplomats are reported to be working to define areas that will fall under the partial truce and to set up monitoring mechanisms.
The UN's Syria envoy has said he hopes the agreement will lead to a resumption of peace talks which collapsed earlier this month in Geneva.
Syria's top opposition grouping -- the Riyadh-based High Negotiations Committee (HNC) -- said Friday that 97 opposition factions had signed on "to respect a temporary truce" but reiterated that it was only agreeing to an initial period of two weeks.
It said the Syrian government and its allies must not continue attacks on rebel forces "under the pretext of fighting terrorism".
Russia and the United States are on opposing sides of the conflict, with Moscow backing Assad and Washington supporting the opposition, but the two powers have been making a concerted push for the ceasefire to be respected.
Iran, another key Assad ally, has said it is confident the regime will abide by the agreement.
Huddling with his national security advisors in Washington on Thursday, Obama put the onus firmly on the regime and Russia.
"The coming days will be critical, and the world will be watching," Obama said.
US Secretary of State John Kerry has been a major advocate of the ceasefire but others in Washington have been less optimistic about the chances of ending a conflict that has left more than 270,000 dead and forced millions from their homes.
Kerry has warned that Washington is considering a "Plan B" to deal with Syria if the ongoing efforts fail.
He has not detailed the new strategy but officials suggest it could involve increased support and more advanced weaponry for moderate rebels.
Russian Foreign Minister Sergei Lavrov said that for the ceasefire to work, Washington should abstain from talk about "some sort of Plan B, about preparing a ground operation, about the creation of some sort of useless buffer zones".