Polish Prime Minister Donald Tusk has warned of the "real" threat of conflict in Europe, saying that for the first time since the end of World War II the continent has entered a "pre-war era".

"War is no longer a concept from the past. It is real, and it started over two years ago. The most worrying thing at the moment is that literally any scenario is possible. We haven't seen a situation like this since 1945," Tusk said in an interview with the European media grouping LENA on Friday.

"I know it sounds devastating, especially for the younger generation, but we have to get used to the fact that a new era has begun: the pre-war era. I'm not exaggerating; it's becoming clearer every day."

Russia's invasion of Ukraine more than two years ago upended European leaders' sense of unshakeable post-war peace, prompting many countries to gear up weapons production to supply both Kiev and their own militaries.

Former European Council president Tusk, whose country has been one of the staunchest supporters of neighbouring Ukraine, said Friday that if Kyiv lost, "no one" in Europe would feel safe.

Also front of mind for the continent is the potential return of former US president Donald Trump, whose openly NATO-sceptic posture has raised questions over the reliability of American military assistance should he be re-elected in November.

"Our job is to nurture transatlantic relations, regardless of who the US president is," Tusk said in the interview.