Tusk spoke on the occasion of an informal EU summit in Cyprus where EU leaders will also discuss the war in the Middle East, energy measures in response and the Union's next long-term budget.
Europe's "biggest, most important question" is whether the United States is ready to be a loyal NATO partner in case of a Russian attack, Polish Prime Minister Donald Tusk told the Financial Times.
He also called for the European Union to become a "real alliance" in protecting the continent, reflecting growing worries about US President Donald Trump's unpredictable policies and threats towards his European partners.
"For the whole eastern flank, my neighbors... the question is if NATO is still an organization ready, politically and also logistically, to react, for example against Russia if they try to attack," he said in an interview published in the British newspaper on Friday.
Tusk said a potential Russian attack was "something really serious."
"I'm talking about short-term perspectives, rather months than years... For us, it's really important to know that everyone will treat the NATO obligations as seriously as Poland."
EU bloc should reflect on mutual defense clause
Tusk spoke on the occasion of an informal EU summit in Cyprus where EU leaders will also discuss the war in the Middle East, energy measures in response and the Union's next long-term budget.
Tusk suggested that the bloc could also reflect on its mutual defense clause, Article 42.7 of the EU treaty, after the departure of Hungarian Prime Minister Viktor Orban, an ally of Russia.
"What you need if you want to have, not only on paper, a real alliance, is true tools and real power when it comes to defense instruments and mobility of militaries from country to country etc. It's a very practical problem for today," he said.
"This is why my obsession now and my mission is to reintegrate Europe. It means common defense... a common effort to protect our eastern borders."