AFP/Special Correspondent/reuters

BEIRUT/WASHINGTON - 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".

Speaking to reporters at the State Department following a meeting of the National Security Council, Obama said although the cessation of hostilities was scheduled to start in Syria at midnight local time Friday, there are “plenty of reasons for scepticism,” but it could save lives if implemented.

“We don’t expect the violence to end immediately,” he said from a rostrum where he was flanked by Secretary of State John Kerry, Secretary of Defence Ash Carter, Attorney General Loretta Lynch, Chairman of the Joint Chiefs General Joseph Dunford, and Brett McGurk, the special envoy to the coalition against the Islamic State.

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 militants.

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.

Saudi jets on Friday arrived at a Turkish base to join the air campaign against Islamic State militants in Syria only hours before a ceasefire is to take force, local media reported.

Four F-15 jets landed at Incirlik air base in the Adana province in southern Turkey, the state-run Anatolia news agency reported.

The base is already hosting US, British and French war planes taking part in the strikes against IS fighters in Syria. Saudi Arabia's air force had already sent ground personnel and equipment aboard C-130 Hercules military transport planes early this week.

Nearly 100 rebel factions in Syria agreed Friday to abide by a Russian-US ceasefire for two weeks, hours before it was due to come into force, the country's top opposition grouping said.

"Factions of the Free Syrian Army and the armed opposition agree to respect a temporary truce... for two weeks," the High Negotiations Committee (HNC) said in a statement, referring to "97 factions from the opposition".

Members of the 17-nation group backing the process were to meet in Geneva on Friday 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-militant 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.

Meanwhile, the al Qaeda-linked Nusra Front rejected on Friday the cessation of hostilities in Syria due to begin at midnight, and urged insurgents to intensify attacks against President Bashar al-Assad and his allies.

Leader Abu Mohamad al-Golani said in an audio message played on Orient News TV that if Syria's war was not resolved, the consequences would spread to Sunni Muslims in other parts of the region. "O soldiers of Sham (Syria), God honoured you with a great jihad on blessed land so strengthen your resolve and intensify your strikes, and do not let their planes and great numbers (of troops) scare you," he said.

The message came ahead of a cessation of hostilities due to take effect at midnight under a U.S.-Russian plan, but which excluded Nusra Front and Islamic State fighters. Golani said the truce plan was a plot which signalled the start of a political process that would keep the Assad government in power. "The successful revolution is one which uproots the old regime with all its institutions," he said.

A victory for Alawites and Shi'ites - a reference to Assad and his allies including Iran - would mean a "shift of the battle to the Arabian Peninsula in less than a decade", he said. Syria's main Saudi-backed opposition body has accepted the proposed cessation of hostilities.