Russian dynasty: It is believed that the GRU had its own spies at the base attacked near Poland

Ukrainian and western intelligence officials believe Russian spies have provided information from a Ukrainian base that was attacked Sunday, with three Britons believed to be among those killed, writes Periscope.

Officials are investigating whether an insider appeared as a foreign recruit for Ukraine was actually working for Russia's spying agencies at the Yavoriv base.

Mirror has learned that Western and Ukrainian agencies believe Russian spy agencies GRU and SVR had transformed foreign fighters into the base.

One of the 1500 foreign recruits there who was supposed to fight for Ukraine against Russia allegedly sent information and co-ordination to Russia.

A reliable source told the Daily Mirror that a Ukrainian army guard at the base had noticed a man running from the base, which seemed to carry a laptop and a sharp object before the attack took place.

The statement, Mirror understands, is being investigated by Ukrainian intelligence and security service, which doubts that Russia had been a spy within the base.

Three former Special Forces troops were allegedly killed in the deadly air strike near the border with Poland, Mirror reported.

Sources say the three people in question were not part of foreign fighters until the exercise occurred at the already destroyed Yavoriv base, which is just a few kilometers from the Polish border.

It has not yet become known in which unit of special forces they were serving.