By Vanessa Gera | Associated Press

WARSAW, Poland — Western security officials suspect Russian intelligence is behind a plot to place incendiary devices in packages aboard cargo planes bound for North America. North, including one that caught fire in a mail center in Germany and another that ignited in a warehouse in England.

Poland said last month it had arrested four people suspected of being linked to a foreign intelligence operation that carried out sabotage and looking for two more. Lithuanian Prosecutor General Nida Grunskiene said Tuesday that an unknown number of people were being detained in several countries, without providing details.

These events come as Western officials say they are seeing an intensification of tensions. Russia’s hybrid sabotage war against Ukraine’s alliesincluding election disinformation and arson in Europe this year. Several officials said they believed the attacks were the work of Russian military intelligence, the GRU, although Moscow has denied any involvement.

The Polish Internal Security Agency, or ABW, says incidents in Poland, as well as other EU and NATO members, have intensified this year. ABW believes that they are initiated and coordinated by Russian special services. So far, 20 people have been charged in investigations by the public prosecutor’s office, the ABW and the police.

Polish prosecutor Katarzyna Calów-Jaszewska said that investigation focuses on foreign agents carrying out acts of sabotageincluding damage to industrial plants or critical infrastructure such as airports, aircraft and other vehicles, as well as arson using self-combustible packages sent to EU countries and the UK which ignite during road or air transport.

She added that the group had tested a channel to send such packages to the United States and Canada.

The Wall Street Journal was first to report details of the cargo plane incidents.

The U.S. Transportation Security Administration said it has implemented additional security measures in recent months for some shipments of goods destined for the United States.

“We continually adjust our security posture as appropriate and promptly share all relevant information with our industry partners, to include requirements and recommendations that help them reduce risk,” TSA said.

There was no active threat to U.S.-bound flights, according to a U.S. official, who was not authorized to comment publicly.

Dirk Heinrichs, a spokesman for DHL in Germany, said in an emailed statement to The Associated Press that the company could not provide details of the case but was “cooperating fully with the relevant authorities to protect our people, our network and our customers’ shipments. .”

The head of Britain’s domestic intelligence agency, MI5, said last month that the UK was facing a “staggering increase” in assassination attempts, sabotage and other crimes on its soil by the Russia and Iran.

Calów-Jaszewska said on October 25 that packages containing camouflaged explosives had been sent via freight companies to EU countries and Britain to “test the transfer channel for such packages” which were ultimately intended for the United States and Canada.

Incendiary devices in Germany and the United Kingdom both caught fire in July.

One of them was stopping at a DHL logistics center at an airport in the city of Leipzig, according to Thomas Haldenwang, head of German intelligence services. German news agency dpa reported that the connecting flight containing the package, from one of the Baltics, was delayed in Leipzig and was on the ground when it caught fire and set fire to a cargo container.

British counter-terrorism police are investigating whether Russian agents were behind an incendiary device in a package that caught fire at a DHL warehouse in Minworth, near Birmingham, central England, on July 22. The incident, first reported by the Guardian newspaper and German television channels, was similar to that in Germany.

Polish newspaper Gazeta Wyborcza also reported that a fire was reported in a courier truck near Warsaw.

“We are observing aggressive action by Russian intelligence services. In particular, Russian espionage and sabotage in Germany are increasing, both quantitatively and qualitatively,” Haldenwang told the Budestag, or German parliament, last month while discussing the Leipzig incident.