BRUSSELS - Transatlantic relations have become more natural since Joe Biden took over as U.S. president after a sometimes "more awkward" period with Donald Trump, the leader of Washington's NATO ally the Netherlands said on Monday.

Dutch Prime Minister Mark Rutte spoke as he arrived at Biden's first summit with leaders of the Western alliance that Trump regularly lambasted and threatened to leave.

"With Joe Biden... there is a clear understanding of the necessity of NATO," Rutte told reporters.

"I also was able to work with Trump. Of course, it was also a bit more awkward sometimes but in the end we were still able to come to close agreements... But with Joe Biden, it's more natural again."

Trump accused NATO allies of not spending enough on their own defence and excessively burdening the United States.

In a clear change of tone, Biden said on arriving at NATO's headquarters in Brussels on Monday that the alliance was "critically important" to Washington and that he saw the mutual security guarantees as a "sacred obligation".

(Writing by Gabriela Baczynska; Editing by Andrew Heavens) ((gabriela.baczynska@thomsonreuters.com; +32 2 287 68 39; Reuters Messaging: gabriela.baczynska.thomsonreuters.com@reuters.net))