US President Donald Trump publicly endorsed NATO's one-for-all-and-all-for-one mutual defence pact during a keynote speech in Warsaw on Thursday, seeking to ease allies' concerns that the US safety net had frayed.

"The United States has demonstrated not merely with words, but with it actions, that we stand firmly behind Article Five," Trump said, referring to the mutual defence commitment.

But he said that more defence spending was needed on the eastern side of the Atlantic, "for its own protection, and you know this. Everybody knows this. Everybody has to know this. Europe must do more."