NATO has upped its "vigilance" amid an increase in "Russian intelligence activity" across its member countries, the military alliance's secretary-general acknowledged on Tuesday
Asked about recent suspected Russian sabotage and arson attempts targeted at warehouses, shopping centres and households across EU and NATO countries, Stoltenberg refrained from directly pinning responsibility on the Kremlin.
But he clearly indicated the alliance was monitoring the events and underlined the importance of criminal investigations at national level.
"We have seen increased Russian intelligence activity across the alliance, therefore we have increased our vigilance, we have our own services who are closely monitoring what the Russians are doing," Stoltenberg - who is due to step down in October after a decade at NATO's helm - said.
"It is important that also nations are taking action (with) the arrests and the legal processes that have now started in the UK, in Germany, in Poland and in other countries," he added.
"We need to be ready and we need to share intelligence (...) to counter these different types of hybrid acts aimed at undermining the ability of NATO allies to provide support to Ukraine."
Russian attempts to 'intimidate'
Over the weekend, a New York Times report citing US and allied security officials claimed some of the recent hybrid attacks - notably attempted arson - in Europe were the work of the GRU, Moscow's military intelligence arm.
The incidents include recent arson attempts on sites across Poland which led to nine individuals being arrested on suspicion of Russian sabotage last Tuesday.
Speaking to Polish broadcaster TVN24 last week, Polish Prime Minister Donald Tusk said the suspects - allegedly Ukrainian, Belarusian, and Polish citizens - had "engaged directly on behalf of the Russian services in acts of sabotage in Poland," including physical assault, arson and attempted arson.
Tusk said the attacks included an attempt to set fire to a paint factory in the western Polish city of Wroclaw, as well as other recent incidents in Lithuania, Latvia, and potentially Sweden.
A recent arson at an Ikea warehouse in Lithuania is now suspected of being linked to the Kremlin's operations.
Speaking to local media last Wednesday, Latvian Prime Minister Evika Siliņa also warned of possible Russian operations in the country, noting that "arson is one of the most typical forms of diversion for Russian special services."
In April, two German-Russian nationals were arrested in Germany for planning bombings and arson attacks against critical infrastructure and military bases.
Authorities in the United Kingdom, a non-EU member state but a NATO ally, also arrested two men in late April after an arson attempt on a Ukraine-linked business on an industrial estate in east London.
Speaking ahead of a meeting of EU defence ministers in Brussels on Tuesday, the Netherlands' defence minister Kajsa Ollongren also raised the alarm: "What we're seeing now in several European countries is Russia trying to destabilise us, also to intimidate us."
"We are aware of this new way of working," she added. "And, so, yes, we are vulnerable. I think all of us are. We have vital infrastructure, we have undersea bed infrastructure, we have electricity supplies, water supplies (...) We are vulnerable but we can also protect ourselves if we cooperate between partners, with industry, and with our security services," she added.
The bloc's top diplomat Josep Borrell said there was no evidence yet to incriminate Russia for such attacks.
