Intense Russian air strikes battered rebel bastions across Syria today, 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 to meet in Geneva today to work out further details of the so-called "cessation of hostilities", which is 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," 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 overnight 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, he 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.
