Russian President Vladimir Putin on Friday said that the peace process in Syria would be "complicated" but that there were no other ways of ending the conflict, ahead of the scheduled start of a ceasefire.

"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 in televised comments.

The Russian leader, however, insisted that there would be no let-up in Moscow's bombing campaign against Islamic State (IS) group militants, the Al-Nusra Front and other "terrorist groups" in Syria after the truce deal enters into force.

"I want to underline again that IS group, Al-Nusra and other terrorist groups that have been designated as such by the United Nations Security Council are not included (in the ceasefire deal)," Putin said.

"The decisive fight against them will, without doubt, be continued."

The landmark "cessation of hostilities" pact brokered by Russia and the United States is due to take effect at 2200 GMT on Friday in a move that marks the biggest diplomatic push yet to help end Syria's violence.

Both President Bashar al-Assad's regime and Syria's main opposition grouping have agreed to the deal -- which allows fighting to continue against IS group and other militants.

Putin said that Moscow was already receiving confirmation from the warring sides that they were willing to abide by the ceasefire and that it would go into effect as planned.

As the clock ticked down towards the ceasefire deadline, Britain-based monitor the Syrian Observatory for Human Rights said Russia was continuing to bombard rebel bastions across Syria, but Moscow insisted its targets were "terrorist organisations".

Russia has been flying a bombing campaign in Syria since September in support of forces loyal to its long-standing ally Assad.

Officials in the West have expressed fears that Moscow and Assad could use the fight against "terrorist groups" in Syria as a loophole to continue attacks against those battling the regime in Damascus.

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