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Initial reports suggest the £7 million aircraft was brought down by Syria’s Turkish-backed National Army. There are also reports the revolutionary faction had destroyed two Syrian government tanks during violent clashes. The latest developments bring NATO member Turkey and Russian-backed Syria close to the brink of direct confrontation a day after President Recep Tayyip Erdogan warned the launch of a new military operation was imminent.

Russian warplanes have continued to bombard rebel-held towns in northwest Syria again as Turkish artillery supported insurgent attacks elsewhere across the region.

In Geneva, the United Nations refugee chief called for a halt to the fighting to allow hundreds of thousands of trapped civilians to move to places of safety.

Russia and Turkey back opposing sides in the nine-year-old conflict. The Kremlin, which has supported Mr al-Assad's push with air strikes against the rebel militia, said a clash between Turkish and Syrian forces would be a "worst-case scenario" and Russia would work to prevent the situation from worsening.

Syrian troops supported by Russian warplanes and special forces have been battling since December to eradicate the last rebel bastions in Idlib and Aleppo provinces in what could be one of the final chapters of the bloody civil war.

But has Turkish troops took up strategic positions across Idlib, Mr Erdogan said he would not leave the region "to the devices of the Assad regime”.

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