"Russia is cooking up something big this year," Poland's top diplomat said.
Speaking at the Council on Foreign Relations in New York alongside his Romanian counterpart Oana Țoiu, Sikorski noted he could not name his sources but stressed there were many.
He suggested President Vladimir Putin may need "a pretext for mobilisation," raising the possibility of a false-flag operation to convince Russians they are under attack.
Sikorski said Belarusians were "very worried" about being drawn into the war.
He also pointed to Russia's potential interest in a land corridor to Kaliningrad through Lithuania and to large Russian-speaking populations in border areas of Estonia and Latvia.
Sikorski called a full-scale invasion "crazy" for Russia, predicting instead a less clear-cut escalation that would also complicate any NATO Article 5 response.
He said Poland would defend itself if necessary, as it has done before, and linked recent Russian strikes on Polish border crossings to a wider strategy of cutting Ukraine off from Western supplies.
Despite economic strain, Sikorski said Moscow could sustain its war for another year or two unless Kyiv destroys over half of Russia's refining capacity.
Moscow still has options such as mobilisation, a harsh winter and continued strikes on Ukraine's energy infrastructure—though he added Kyiv has "cards to play" too.
On a potential permanent US base in Poland, he said it would eventually house one brigade, alongside roughly 7,000 rotational US troops currently in the country.
According to Sikorski, around 10,000 American soldiers would be enough to reassure Poland without "threatening" Russia.
He voiced hope US President Donald Trump would honour his commitment on the matter.
(ał)
Source: PAP