Polish authorities have arrested four people over incendiary devices which caused fires in countries including the UK.

They believe Russia is plotting to attack transatlantic flights with parcels that are “containing camouflaged explosives” which are sent via courier firms, and they are designed to “spontaneously” explode.

There has been dispatches of flammable packages which has been sent via cargo planes in Europe and intelligence suggests Russia could be preparing to conduct similar operation in North America, the Washington Post and the Wall Street Journal have reported separately.

International investigations have revealed they fear the flammable parcels are “test runs” before Russia commits horrendous terrorist acts.

In July a parcel caught fire at a depot in a DHL depot in Birmingham, UK counter terrorism officials are investigating if Russian intelligence planted the device.

Another incendiary device caught fire at another DHL facility in Leipzig, Germany and parts of a shipping container was burnt.

Pawel Szota, head of Poland’s foreign intelligence agency has firmly blamed Russia for these acts of terrorism.

He told The Wall Street Journal, “I’m not sure the political leaders of Russia are aware of the consequences if one of these packages exploded, causing a mass casualty event.”

Katarzyna Calów-Jaszewska, of Poland’s National Prosecutor’s Office, said, “During the proceedings, four people involved in the activities of the revealed sabotage and diversion group of an international nature were detained, charges were announced, and then temporarily arrested.”