NATO foreign ministers meet in the US capital this week for a low-key celebration of the alliance's 1949 founding treaty, with US complaints over weak European military spending firmly on the agenda.

US President Donald Trump has repeatedly accused NATO allies -- and economic powerhouse Germany in particular -- of freeloading on America's military muscle and reportedly threatened to "go it alone" if Europe does not step up.

All NATO allies agreed to move towards spending two percent of GDP on defence by 2024, but last month Berlin announced that its own figure was set to fall in the coming years, from 1.37 percent in 2020 to just 1.25 percent in 2023.

The news infuriated Washington, and Stoltenberg said Berlin must live up to commitments it had signed up to at a summit in 2014.

"I expect Germany to make good on the pledge Germany made together with all other NATO allies," Stoltenberg told reporters.

"I expect them to meet spending commitments, and they have submitted to NATO a national plan where they outline how Germany will increase defence spending in real terms by 80 percent over a decade."

- Germany warns not to test 'unity' -

German Foreign Minister Heiko Maas, speaking in New York ahead of the NATO meeting, said that Berlin had already boosted defence spending by almost 40 percent since 2014 and was one of the alliance's top troop contributors.

"We are grateful for our partnership within NATO, which has enabled us Europeans to enjoy security, stability and prosperity over the last 70 years," he told the American Council on Germany.

"But public debates about burden-sharing in NATO are generating uncertainty -- at a time when Russia is trying to test our unity again and again," he said.

"Europeans know that we need to assume greater responsibility for our security. It lies in our own interest."

In 2018, only seven of NATO's 29 member states hit the two percent target.

Stoltenberg, whose mandate as secretary general was extended by two years to 2022 last week, insists that away from Trump's fiery rhetoric, the US is fully committed to NATO, stepping up its investment of troops and resources in Europe.

And in a sign of NATO's enthusiasm to keep the US on board, Stoltenberg reiterated the alliance's plans to invest more than $260 million (232 million euros) in a facility in Poland to support US forces.

The storage and maintenance facility will allow equipment to be "pre-positioned" as part of NATO's efforts to step up its ability to counter the threat posed by Russia.