US ambassador to Germany Richard Grenell on Sunday criticized Germany's pledge to incrementally raise its defence spending to 1.5 per cent until 2024 in an interview with German Sunday newspaperWelt am Sonntag.

"Germany's NATO promise to raise defense spending to 1.5 percent is not enough," Grenell said. "Again, it is not US standards that have to be met here, but NATO commitments that Berlin has already agreed to. The US is simply reminding its great German ally that now is not the time to undercut or weaken NATO."

Defence spending is a contentious subject that has strained relations between Washington and Berlinsince Donald Trump assumed the US presidency in 2016. Trump has repeatedly accused the German government of failing to meet its defence spending commitments.

Under Trump, the White House has warned NATO allies that it would "moderate" its commitment to the alliance if member states failed to spend more for defence.

"The US is totally committed to NATO," Grenell said. "Donald Trump has made that clear, but he also seeks to improve it. And what is wrong about wanting to fix something that you believe in?"