For US Defense Secretary Jim Mattis, the next few days will be a reassurance tour with a twist.

As much he'll tell allies the US is committed to NATO, he also is expected to visit Europe with a hand out, hoping to secure bigger military spending commitments and greater assistance in military campaigns in Afghanistan and elsewhere.

Mattis also will field questions on national security adviser Michael Flynn's resignation over his pre-inauguration discussions with Russia, and what the change means for US policy on Moscow.

Speaking to reporters on the way to a NATO defense ministers' meeting, Mattis said Flynn's departure "has no effect at all" on him.

"Frankly, this has no impact," he said. "I haven't changed what I'm heading there for. It doesn't change my message at all. Who is on the president's staff is who I will work with. And so, you know, it's full speed ahead."

Mattis also reinforced earlier comments he's made on NATO's importance.

Calling it "the most successful military alliance in history", he told reporters on the plane that "our commitment remains to NATO". But he said allies need to discuss increasing their military funding to the benchmark goal of 2 percent of gross domestic product.

After spending his first official trip telling Asian allies they wouldn't be abandoned, Mattis is on a similar journey in Brussels and Munich this week, where he will have to address the mixed messages from President Donald Trump on the value of NATO before and after he took office.

"I think there will be a lot of reassuring words spoken over the next week," said Derek Chollet, a former senior defense official who is now senior adviser at the German Marshall Fund.

Wednesday marks Mattis' first NATO meeting as defense secretary. During his Senate confirmation hearing last month, the retired Marine general said he wanted the US to "maintain the strongest possible relationship with NATO".

With remarks such as those, Mattis has distanced himself from Trump's criticism of the military alliance and threats that the U.S. might not defend allies that don't fulfill their financial obligations as NATO members.

Mattis, however, is aligned with Trump's call for the 27 NATO members to meet the military spending requirement. Only a few are currently hitting the 2 percent mark, while the US spends more on its armed forces than all the others combined. Washington also foots more than 22 percent of NATO's commonly funded budget.

The US also would like to see an increased NATO commitment in Afghanistan, where forces have been fighting the Taliban for more than 15 years.