Poland's prime minister has said the country was preparing for "various" scenarios and that the coming months could be "critical" concerning the threat from Russia.
Donald Tusk was responding to reports, citing US intelligence, that Moscow was planning an armed "provocation" in Poland to test Nato's resolve.
"I don't mean to scare anyone but the coming months may truly be critical, also due to the changing nature of the war. These concerns are particularly palpable in the Baltic states," Tusk told reporters on Friday.
Polish news outlet Onet reported that sources close to President Karol Nawrocki had said the US had issued several warnings to Warsaw about a plot to attack the country.
The White House and US State Department did not respond to the BBC's requests for comment.
Under the purported plans, also published by the Telegraph in the UK on Friday, Polish infrastructure could be targeted by missiles or drones, or soldiers could be sent into the Nato state.
The reports suggest the aim would be to place pressure on Ukraine's western allies to suspend aid to the country as it continues to fight against Russia's full-scale invasion, launched in 2022.
Asked about the reports, Tusk said: "Let's not be afraid, we are preparing for various situations, but we cannot ignore them... We are aware of the threats, also thanks to information from our allies."
Nawrocki is set to attend a Nato summit with leaders of other members of the defensive alliance in Turkey next week.
Secretary General Mark Rutte has said the gathering will show Europeans are heeding US President Donald Trump's longstanding calls to increase their defence spending, while leaders are also expected to pledge to continue funding weapons for Ukraine.
Tusk told the Financial Times in April that Russia could attack a Nato member state in "months".
In late June, his Deputy Prime Minister Radek Sikorski told CBS News that he would not rule out a Russian "false flag" operation in the next two years to justify an attack on a Nato state.
Baltic states have previously acknowledged concerns over their vulnerability to a Russian attack. Latvian media reported in June that its intelligence services had warned Moscow was planning military provocations in the region, or in Poland.
Lithuania's ambassador to Nato said on Thursday that Russia was more likely to resort to hybrid warfare - such as missile or drone incursions - than a conventional military attack, according to the country's public service broadcaster LRT.
Nato's Article 5 states that an armed attack against one member will be considered an attack against all, for which they will come to each other's defence.
