Saudi Arabia on Sunday accused President Bashar al-Assad's regime and its ally Russia of "ceasefire violations" in Syria.

"There are violations to the ceasefire from Russian and (Syrian) regime aircraft," Foreign Minister Adel Jubeir told reporters in Riyadh.

"We are discussing this with (the 17-nation) Syria Support Group," co-chaired by Russia and the United States, said Jubeir.

Russia, which has waged a five-month bombing campaign to support Assad, blamed "moderate" rebels, Turkey and militants for nine ceasefire violations.

But "on the whole, the ceasefire regime in Syria is being implemented," Lieutenant General Sergei Kuralenko, head of Moscow's coordination centre in Syria, was quoted as saying by Russian news agencies.

The Syrian Observatory for Human Rights has said that several air strikes hit central and northern Syria on Sunday.

Warplanes, believed to be either Syrian or Russian, bombed seven villages in the provinces of Aleppo and Hama, the monitor said.

A ceasefire brokered by Moscow and Washington took effect at midnight on Friday, but the Riyadh-based opposition and Russia have reported several breaches from opposing sides.

The ceasefire agreement does not include territory held by the Islamic State jihadist group and Al-Qaeda-affiliated Al-Nusra Front, which together control more than half of Syria.

Russia said Saturday it had halted bombing in all areas covered by the truce.

But it has vowed to keep striking IS and Al-Nusra and other "terrorist groups".

It was unclear if Sunday's raids hit areas covered by the truce.

But Jubeir said that Russia was targeting Syria's "moderate opposition" groups.

UN special envoy Staffan de Mistura was "in contact with the Russians and the Syrian regime on this matter to reach an agreement that would limit or end military operations against the moderate Syrian opposition and focus instead on Daesh (Arabic acronym for IS) and Al-Nusra," Jubeir said.

"Things will become clearer in the coming days on whether the regime and Russia are serious or not about the ceasefire," he said.

Jubeir added countries supporting the Syrian opposition had an "alternative" without Assad if the truce failed, referring to US State Secretary John Kerry's comments on Tuesday about a "Plan B".

Russian Deputy Foreign Minister Mikhail Bogdanov said Riyadh's statement on the existence of a Plan B ran counter to two UN Security Council resolutions, Russia's Interfax news agency reported.

"We all said there was no 'Plan B'. We must implement what we have decided together," he said.

Saudi Arabia is a main supporter of the Syrian rebels battling Assad's regime since 2011.