Intelligence on Russian sabotage threat prompted increase in security at US military bases in Europe

US military bases across Europe were placed on a heightened state of alert last week for the first time in a decade after the US received intelligence that Russian-backed actors were considering carrying out sabotage attacks against US military personnel and facilities, multiple sources familiar with the matter told CNN.

The intelligence the US received suggested that Russia had included US bases and military personnel as options to attack via proxies, the sources said — similar to plots that have been carried out or disrupted across Europe in recent months.

In April, two German-Russian nationals were arrested for allegedly plotting bomb and arson attacks on targets including US military facilities on behalf of Russia.

The intelligence, which the US received within the last two weeks and has not been previously reported, was deemed alarming enough to implement additional safety protocols, the sources said. Several US military bases in Europe raised their alert level to Force Protection Condition “Charlie,” which “applies when an incident occurs or intelligence is received indicating some form of terrorist action or targeting against personnel or facilities is likely,” according to the US Army.

A senior NATO official said on Tuesday that the alliance had “significantly increased” its intelligence sharing around “Russia’s campaign of covert sabotage activities” in Europe, which have become increasingly brazen and aggressive in recent months amid elections across the West — a “prime opportunity” for Russia to try to undermine public support for Ukraine, the official said

US European Command declined to comment directly on what caused the force protection change last week. But a spokesperson, Cmdr. Dan Day, said that “our increase in vigilance is not related to any one single threat, but due to a combination of factors potentially impacting the safety and security of US forces in the European theater.”

The threat of Russia-backed sabotage efforts has become particularly acute in recent months, amid a series of arson attacks and bombing plots across Europe—including fires and explosions in Riga, Latvia; London; Warsaw, Poland; Prague, the Czech Republic; and Paris — that European security officials have linked to Russia.

In London in March, several men were charged with working with Russian intelligence services to set fire to a Ukrainian-linked warehouse. Poland is investigating whether an arson attack that destroyed Warsaw’s largest mall in May was connected to Russia and has arrested nine people in connection with Russia-linked acts of sabotage, the prime minister said in May. And French authorities last month detained a Russian-Ukrainian man who was allegedly building bombs as part of a sabotage campaign orchestrated by Moscow.

By outsourcing the attacks to local actors, Russia likely believes it can wage a hybrid war that falls below the threshold of armed, state-on-state conflict, officials say. But a senior NATO official said the sabotage campaign is getting increasingly brazen and aggressive.

“What we’re seeing now is a more concerted, more aggressive effort, than what we’ve seen certainly since the Cold War,” the official said on Tuesday. “We’re seeing sabotage, assassination plots, arson — real things that have cost human lives.”

This story has been updated with additional information.

CNN’s Katie Bo Lillis contributed reporting.

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