Two Russian airmen killed in Syria on Friday were shot down with American weaponry, the Interfax news agency said Sunday, quoting a Russian military source.
It said insurgents from the Islamic State group hit the airmen's Mi-25 assault helicopter with a U.S.-made TOW heavy anti-tank missile, a weapon that uses guidance from a ground station.
The high-tech system is in service with the U.S. military.
"According to information we have, the terrorists used an American TOW," Interfax quoted the unnamed source as saying.
The Islamic State group on Saturday claimed responsibility.
Amaq, a news agency that disseminates the group's propaganda, posted online video purporting to show the incident.
The video shows a flash of light at the helicopter's tail, and the rear rotor unit appears to fall away. The helicopter then goes into a spin before crashing, to cries of "God is greatest" in audio accompanying the images.
As the aircraft goes down, a second helicopter passes by. It is of similar appearance to the stricken Mi-25.
Interfax quoted the military source as saying the downed helicopter had run out of ammunition and was returning to base.
Russian newspapers reported the alleged U.S. link without saying how the Islamic State group -- which Washington is fighting in Iraq and Syria -- might have obtained contemporary American weaponry.
In some respects, the narrative mirrors Western allegations that Russia supplied the missile that shot down Malaysia Airlines Flight MH17 over eastern Ukraine in July 2014.
However, the Islamic State group is known to have obtained U.S.-made vehicles and weapons when it seized territory from Iraq's security forces in 2014.
The Islamic State declared a "caliphate" straddling parts of Syria and Iraq after capturing Mosul, Iraq's biggest northern city, in June 2014. Since then, the group has been linked to attacks in Ankara, Brussels, Paris and elsewhere.
With the two latest casualties, Russian authorities have reported the deaths of 13 military personnel in Syria, Interfax said.
This article was written by Alastair Wanklyn from The Japan Times and was legally licensed through the NewsCred publisher network.