Warplanes have unleashed air strikes on the suburbs of the Syrian capital Damascus and near the rebel-held city of Aleppo, hours before a ceasefire brokered by the US and Russia was due to go into effect.
The barrage came as the main Syrian opposition and rebel umbrella group said dozens of factions - 97 groups in all - have agreed to abide by the ceasefire, which is due to begin at midnight local time across the war-ravaged country. The High Negotiations Committee, or HNC, said a military committee has been formed to follow up on the ceasefire.
The Britain-based Syrian Observatory for Human Rights said the warplanes in Friday's strikes were believed to be Russian. The Kremlin did not comment on the latest developments but denied allegations that the Russian air force bombed civilian positions east of Damascus the previous day.
The rebel-held Damascus suburb of Douma was hit 26 times on Friday, the observatory said, along with other areas east of the capital. The monitoring group said the air raids were conducted as the Syrian government's artillery shelled the area, which is a stronghold of the Army of Islam rebel group.
Mazen al-Shami, an activist based in the area, said the warplanes were Russian, adding that they carried out some 60 air raids on Friday as well. He said 25 strikes targeted Douma. "The air raids intensified after the revolutionary factions said they will abide by the ceasefire," Shami said.
The observatory also reported dozens of air strikes north of the northern city of Aleppo, which has been under attack by troops and pro-government militias for weeks.
Late on Thursday US president Barack Obama expressed hope that the ceasefire in Syria will lead to a political settlement to end the civil war and allow a more intense focus on battling Islamic State.
He said he does not expect the truce to immediately end hostilities, after years of bloodshed between forces loyal to Syrian president Bashar Assad and rebels who want to end his reign.
Announced just this week, the ceasefire is a "test" of whether the parties are committed to broader negotiations over a political transition, a new constitution and holding free elections, Mr Obama said. He said Syria's future cannot include Assad as president, which is a chief point of contention with Russia and Iran, who support the Syrian leader.
"We are certain that there will continue to be fighting," Mr Obama said, noting that IS, the Nusra Front and other militant groups are not part of the negotiations and the truce.
Mr Obama put the onus on Russia and its allies - including the Assad government - to live up to their commitments under the agreement. The elusive ceasefire deal was reached only after a months-long Russian air campaign that the US says strengthened Assad's hand and allowed his forces to retake territory, altering the balance of power in the Syrian civil war.
"The world will be watching," Mr Obama said.
In Moscow, Russian president Vladimir Putin said his country will keep hitting "terrorist organisations" in Syria even after the truce is implemented.