The United States, regardless of who is president, will work with Ukraine and the EU for security if "we all take the challenges seriously," Polish Prime Minister Donald Tusk said.

At a press conference with Ukrainian President Volodymyr Zelenskyy in Warsaw on Wednesday, Tusk said American aid to Ukraine, the U.S. presence in NATO, Polish friendship and cooperation with America are "the pillars of global security, global order and Polish security."

He said the U.S. aid is unprecedented, and for Ukraine it is absolutely one of the key conditions for successfully countering Russian aggression.

"On the other hand, European countries, Poland and the EU as a whole should not get hung up on interpretations and speculations about this or that statement by President Trump, but should face a task, and it is quite simple. America, and this does not only depend on who is president, but especially the America of the Trump presidency, will work with Ukraine and with the EU for our security, if we all take it seriously," Tusk said.

He said he had the opportunity to work quite closely with Donald Trump for many years during his first presidential term.

"I do not want to present myself as an expert on President Trump's policies, but my experience tells me quite clearly that we must take seriously the statements of the new U.S. administration, but above all, we must take seriously ourselves," Tusk said.