Syria's Assad blames Israel over downing of Russian plane

"This unfortunate incident was the result of Israeli arrogance and depravity," the Syrian leader said, offering his condolences in a letter to his Russian counterpart for the death of 15 Russian crew members killed in the incident over Syria on Monday.

"We are determined that such tragic events will sway neither you nor us from continuing the fight against terrorism," he continued in the letter published by the official Sana agency.

The Russian plane was downed by Syria's Russian-made S-200 air defence system and all aboard were killed.

The Russian military has accused Israeli pilots of using the Russian plane as a cover, "exposing it to fire from Syrian air defences".

Israel said it had targeted a Syrian military facility where weapons manufacturing systems were "about to be transferred on behalf of Iran" to Lebanese Shia group Hizballah.

It was the worst 'friendly fire' incident between Moscow and the Syrian regime since Russian forces intervened in the country in late 2015 to support Assad whose grip on power had been weakened by rebels and jihadist fighters.

Russian President Vladmir Putin described the incident as the result of "tragic accidental circumstances," warning Israeli Prime Minister Binyamin Netanyahu against carrying out such operations in the future and pledging to beef up security for Russian forces in Syria.

Netanyahu on Tuesday expressed his "sorrow" to Putin in a phone call, offering to assist Moscow in the investigation.

However he insisted the Russian plane had been felled by "extensive and inaccurate Syrian anti-aircraft (fire)".

In recent months Israel has increased its attacks in Syria against the regime or its ally Iran.