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 U.S. President Donald Trump’s unpredictable policies and ​threats towards his European partners.

“For the whole eastern flank, my ​neighbours…  the question is if NATO is still an organisation ⁠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.”

Tusk was speaking 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 defence 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 defence 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 defence… a common effort ​to protect our eastern borders.”