"Now is the time to invest because you can't make an assumption that America's presence will last forever," Hegseth told reporters alongside his Polish counterpart in Warsaw.

He was visiting Poland on his maiden trip to Europe as a top official in President Donald Trump's administration. Trump has repeatedly called on NATO members to increase their military expenditure.

Hegseth praised "model ally" Poland -- a country on NATO's eastern flank that neighbours both Russia and Ukraine -- for its defence spending.

Warsaw aims to spend 4.7 percent of its annual economic output on defence in 2025, well above the alliance's current minimum level of two percent.

Trump has previously said that should be more than doubled to five percent of gross domestic product (GDP).

Poland's Defence Minister Wladyslaw Kosiniak-Kamysz said Europe's current arms production capacity was "not enough".

"Europe needs to wake up," he added. "Europe needs to invest in the arms industry."

Hegseth defended Trump's efforts to launch negotiations on the Ukraine war with Russia's leader.

And when asked if he trusted Russian President Vladimir Putin to live up to any potential agreement, Hegseth told reporters, "You don't have to trust somebody in order to negotiate with them".

Trump on Wednesday blindsided Ukraine and Washington's European allies by agreeing to launch talks on resolving the Ukraine conflict in his first publicly announced phone call with Putin since returning to power.

The same day, Hegseth said it was not realistic for Ukraine to regain all its land, or become a member of NATO.

On Friday, Ukrainian President Volodymyr Zelensky said he was ready for direct talks with Russia once Kyiv had reached a common position with the United States and Europe on how to end the nearly three-year war.