NATO chief Jens Stoltenberg on Tuesday said the top priority for the transatlantic alliance is to increase defence spending, as demanded by US President Donald Trump.

"Regardless of language, the most important thing is that we increase defence spending and that is exactly what we are doing," Stoltenberg said when asked about NATO's response to Trump's calls for it to do more to share the burden with Washington.

On the campaign trail and in his first days in office, Trump appeared to put in doubt the near 70-year US security guarantee for NATO which he dubbed "obsolete" while accusing some allies of not paying their way.

His remarks caused consternation among the allies who, stung by Russia's intervention in Ukraine, had agreed in 2014 to increase defence spending to two percent of national economic output by 2024, reversing years of cuts.

Stoltenberg said all 28 allies had agreed that commitment and they reaffirmed it last year at a Warsaw summit.

"That has been my top priority and I have raised it in all the meetings that I have had," he told a press briefing before a NATO defence ministers meeting in Brussels Wednesday and Thursday.

"In 2015, we stopped the cuts and in 2016 ... we made the first significant step in increasing defence spending by 3.8 percent" or $10 billion, he added.

He said that in two phone calls with Trump, the new president "strongly expressed his strong commitment to NATO ... but in both calls he underlined fair burden sharing."

"Those that spend less than the two percent have to meet the two percent target and I agree with him," he said.

The NATO defence ministers meeting is being closely watched as it is the first for new US Defence Secretary James Mattis, with his peers anxious to hear first hand what the new administration plans.

Mattis, who served several years in NATO, has appeared more conciliatory on the future of the alliance and relations with Russia compared with the president.

Stoltenberg downplayed the issue when asked about possible differences or tensions within the new US administration.

"The important thing for me is that the president, the secretary of defence, the secretary of state have all conveyed the same strong message about NATO," he said.

"They stated clearly that the United States stays strongly committed to NATO and the transatlantic bond."