Russia's malign hybrid activities in NATO countries are a source of deep concern, the presidents of Romania, Poland and Latvia said on Tuesday in a statement released to coincide with a meeting of eastern members of the alliance.
Countries on NATO's eastern flank say that Russia is using tactics ranging from sabotage and cyberattacks to illegal migration to destabilise them due to their support for Ukraine.
Russia has repeatedly denied accusations that it is behind such activities.
"We are deeply concerned about Russia's recent malignant hybrid activities on Allied territory, which constitute a threat to Allied security," the presidents said in the statement.
"We will act individually and collectively to address these actions, boost our resilience and continue to coordinate closely to ensure that the Alliance and Allies are prepared to deter and defend against hybrid actions or attacks."
Polish Prime Minister Donald Tusk said on Tuesday that 10 people had been arrested in Poland in recent weeks for acts of sabotage.
Czech Prime Minister Petr Fiala said on Monday that Russia may have been behind an attempted arson attack on Prague city buses last week.
The B9 group of NATO eastern-flank states is holding a summit in the Latvian capital Riga, with Romanian President Klaus Iohannis, Polish President Andrzej Duda and Latvian President Edgars Rinkevics acting jointly as chairmen.
The meeting comes ahead of a NATO summit in Washington in July.
"We expect that decisions at the Washington Summit will promote Ukraine's full interoperability with NATO and build a tangible bridge for its NATO membership," the three presidents said.
"On the road to the Washington Summit we stand committed together with Allies to agree on a substantial support package, including on a comprehensive NATO role in supporting Ukraine's inherent right for self-defence and a pledge to ensure sustainable long-term financial support."
