Trump declares NATO stronger with allies 'agreeing to pay'

US President Donald Trump headed into a controversial summit with Russia's Vladimir Putin on Monday insisting that the NATO alliance had never been stronger or more unified.

"NATO's never been stronger. NATO has never been more together," he said at a breakfast meeting with the Finnish President Sauli Niinisto. US allies "are now agreeing to pay" their fair share, Trump added.

Trump's comments come after he demanded last week that NATO allies "reimburse" the US for defence costs.

Trump touched down in Brussels last wednesday aboard Air Force One hot on the heels of a stern warning from the EU's top official that the US should "appreciate" its allies.

The leaders of the other 28 members of the NATO alliance are hoping for a show of unity despite stark transatlantic tensions on a host of issues, most notably Trump's repeated invective against Europe over defence spending.

"Many countries in NATO, which we are expected to defend, are not only short of their current commitment of 2 percent (which is low), but are also delinquent for many years in payments that have not been made," he tweeted while his plane was in the air. "Will they reimburse the US?"

Trump had said before leaving Washington that his meeting in Helsinki with Russian President Vladimir Putin "may be the easiest" part of his upcoming European tour, which saw a trip to the UK and disagreement over the government’s handling of Brexit.

"Frankly, Putin may be the easiest of them all. Who would think?" he told reporters.

He vowed not to be "taken advantage" of by the EU, which he accuses of freeloading by relying on the US for its defence while blocking US imports into the bloc, the world's biggest market.

"It's certainly going to be an interesting time with NATO," Trump told journalists.

Trump's comments differ from his previous position. He had ramped up his rhetoric ahead of the talks - including in three separate tirades on Twitter last Monday and Tuesday - making the summit one of the most difficult in years for the military alliance that has underpinned European security since World War II.