US Secretary of State Marco Rubio has spoken of a defining moment and a "new era" as he travels to Europe for a major speech to the Munich Security Conference.

Rubio will lead the US delegation at the first major global event since President Donald Trump threatened Danish sovereignty with a pledge to annex Greenland.

French President Emmanuel Macron has insisted Europe must prepare for independence from the US, while Nato Secretary General Mark Rutte has stressed that transatlantic bonds are as close and important as ever.

The war in Ukraine, tensions with China and a potential nuclear deal between Iran and the US are also on the agenda as the security conference gets under way.

"The world is changing very fast right in front of us," Rubio told reporters, when asked if his message to Europeans would be more conciliatory than a year ago.