Istanbul (AFP) - Russia's bombing campaign in Syria is a "grave mistake" and its support for the regime will be judged by history, Turkish President Recep Tayyip Erdogan warned on Sunday.

"The steps Russia is taking and the bombing campaign in Syria are quite unacceptable to Turkey," Erdogan told reporters at Istanbul airport before leaving for a visit to France.

"(...) Unfortunately, Russia is making a grave mistake," he said.

Given Moscow's friendly relations with Turkey, its actions in Syria are "worrying and disturbing," Erdogan said.

The strikes will "isolate Russia in the region," he predicted.

Russia and Turkey have long been at odds over Syria, with Moscow emerging as Syrian President Bashar al-Assad's key international backer and Ankara seeing his ouster as the only solution to the conflict.

"What's Russia trying achieve there anyway?" Erdogan asked.

"It intervened because this was what the regime in Syria demanded. But there's no obligation to respond like this every time a regime insists on it," he said.

Erdogan blamed "not only Russia" but also Iran, which was defending Assad and "waging state terrorism" in Syria.

"The countries that cooperate with the (Syrian) regime will be accountable to history," he warned.

Russia, which began the air strikes in Syria last week, insists the operation only targets jihadists.

But Turkey and several of its Western allies have said moderates fighting Assad's regime have also been hit.

Erdogan on Saturday urged Russian President Vladimir Putin to reconsider Moscow's bombing campaign, accusing the Russian military of turning a blind eye to the killing of dozens of civilians.

Turkey has long been lobbying for a "safe zone" free of IS fighters, where some of the two million Syrian refugees hosted by Turkey could go.

The plan has met with only a lukewarm reception from Turkey's partners and its prospects may be further dimmed by Russia's air campaign.

But Erdogan on Sunday vowed to push hard on the idea of a haven.

"It has by no means dropped off our agenda. But there has to be a no-fly zone too. Otherwise, the area could be lethal."

Erdogan said Turkey had spent $7.5 billion (6.69 billion euros) on Syrian refugees -- a burden that, he said, was part of Turkey's "moral duty.