A spokesman for Russian leader Vladimir Putin has said it is “no secret” that US military equipment is being used against Russian forces in both Ukraine and Russia itself.

The statement from Kremlin spokesman Dmitry Peskov comes as the Pentagon has launched an investigation into claims that US armoured cars were used in the raid on the Russian city of Belgorod, around 25 miles from the border with Ukraine.

Footage allegedly taken during the Belgorod attack showed at least three US-made Mine-Resistant Ambush Protected vehicles (MRAPs) being used during the raid. The raid was reportedly carried out by a pro-Ukrainian militia made up of Russian citizens, but Pentagon spokesman Brigadier General Patrick Ryder has said the US government had not “approved any third-party transfers of equipment to paramilitary organizations”.

General Ryder added that Kyiv had also not “requested” any such transfers.

Confirming that the investigation was under way, US Department of State spokesman Matt Miller told reporters: ”As a general policy matter, we have been clear that we don’t support the use of US-made equipment being used for attacks inside of Russia”.

The confirmed use of western-made equipment by anti-Putin militias within Russia would bring the spectre of a direct confrontation between Russia and NATO significantly closer.

Peskov told reporters at a Kremlin briefing earlier this week: “It is no secret that this equipment is being used against our own military… and it is no secret for us that the direct and indirect involvement of Western countries in this conflict is growing by the day.”

Denying accusations that the Belgorod attack had been carried out by Ukrainian special forces, Ukrainian National Security Advisor Oleksiy Danilov told CNN that the insurgents that the fighters “are Russians, it is their country and they have the right to be there”.

He added: “There are some Russians who are on the side of the light and who went to deal with the darkness that exists in Russia now”.