The incidents "are part of an intensifying campaign of activities" Russia is carrying out across the Euro-Atlantic area and NATO allies "express their deep concern over Russia's hybrid actions, which constitute a threat to allied security", NATO said in a statement.

Authorities in the Czech Republic, Estonia, Germany, Latvia, Lithuania, Poland and Britain have recently investigated and charged people in connection with "hostile state activity".

NATO said allies would work together to deter and defend against the hybrid actions and that they would remain steadfast in supporting Ukraine as it struggles to fend off Russia's invasion, now in its third year.

Last month, a 20-year-old British man was charged with masterminding an arson plot against a Ukrainian-linked target in London. Moscow's ambassador Andrey Kelin dismissed claims of links to Russia as "absurd" and "unfounded".

In late March, Czech authorities said they had busted a Moscow-financed network that spread Russian propaganda and wielded influence across Europe, including in the European Parliament.