Nato defence ministers are meeting in Belgium for their first gathering with the new US Defence Secretary James Mattis.
General Mattis is expected to reaffirm US commitment to the defence alliance, after disparaging comments from President Donald Trump.
But he is also expected to deliver Mr Trump’s demand that many Nato countries need to increase military spending.
The meeting comes amid concern over the new administration’s ties with Russia.
It also follows reports that Russia violated a landmark arms control treaty by deploying a new cruise missile, which the Kremlin has dismissed.
Mr Trump’s comments, during his campaign, that the US might not defend allies who do not contribute their “fair share” to Nato have worried many European nations, particularly those near Russia’s border.
He was critical of the Western military alliance, describing it as “obsolete”.
But ahead of Wednesday’s meeting in Brussels, Nato Secretary General Jens Stoltenberg indicated a willingness to meet US demands.
He unveiled figures showing that European members and Canada had between them increased defence spending by 3.8% above inflation last year.
He said: “The most important thing is that we increase defence spending and that is exactly what we are doing.”
Gen Mattis will have strong support from Britain – one of only five Nato countries to meet existing targets for defence spending, says the BBC’s defence and diplomatic correspondent Jonathan Marcus.
But the resignation of Michael Flynn as US national security adviser – and the rumbling row over the Trump team’s contacts with Russia – is likely to cause unease among the 28-nation group, our correspondent adds.
Speaking to reporters on the way to the Nato meeting, Mr Mattis said Mr Flynn’s departure had “no impact” on his role.
He praised Nato, calling it “the most successful military alliance in history”.