The Syrian government has begun releasing 169 militants in exchange for the bodies of five Russians killed after their helicopter was shot down in northern Syria in August, a lawyer says. 

Michel Chammas, a lawyer representing many of the prisoners, said 50 "inmates" including seven women were set free from Adra prison and 84 others from Hama prison on Tuesday.  

“Another 31 inmates at Homs prison were also informed that they are going to be let go but they still haven’t been released. Four others held elsewhere should also be released,” he said.

Chammas declined to comment on the name of the militant groups involved in the swap deal, but a spokesman for the militants said they had charged a “commission” with negotiating the swap.

The so-called Syrian Observatory for Human Rights said that 86 people had been released in the central city of Hama.

The Russian military helicopter was downed in Syria's militant-held Idlib province on August 1, killing all five people on board.

According to the Russian Defense Ministry, the Mi-8 military transport helicopter was shot down after delivering humanitarian aid to the Syrian city of Aleppo as it made its way back to Russia's main airbase in the western province of Latakia.

Since late September 2015, Russia has been assisting Syria, based on an official request from Damascus, in attacks the government is launching against terrorists across the country.  

Syria has been gripped by foreign-backed militancy since March 2011. UN Special Envoy for Syria Staffan de Mistura estimates that over 400,000 people have been killed in the conflict.

Back in 2014, the UN said it would no more update its official death toll for Syria because it could not verify the figures that it received from various sources.