NATO chief Jens Stoltenberg on Thursday stressed the need for solidarity in the alliance after Donald Trump raised the issue of his own commitment to defending other countries in the group.

Trump, the Republican presidential candidate, suggested in a New York Times interview that under his leadership the US could review its relationship with a NATO ally before deciding whether to spring to its defence.

NATO is based on the principle of collective defence, with all members agreeing to defend each other against external attacks.

"I will not interfere in the US election campaign, but what I can do is say what matters for NATO," Stoltenberg said in a statement sent to AFP.

"Solidarity among allies is a key value for NATO. This is good for European security and good for US security. We defend one another."

Trump said that in the event of a Russian attack on the Baltics, NATO's newest members, Washington might assess whether those nations "have fulfilled their obligations to us" before deciding whether to come to their aid.

"If they fulfil their obligations to us, the answer is yes," he said.

A cornerstone of the 28-member trans-Atlantic alliance is its Article 5 commitment that an attack on one member state is an attack on all.

The pledge has only been invoked once in the group's 56-year-history -- in defence of the US after the September 11, 2001 attacks.

"We have seen this in Afghanistan, where tens of thousands of European, Canadian, and partner nation troops have stood shoulder-to-shoulder with US soldiers," Stoltenberg said.

Trump also reiterated to the New York Times that under his presidency, the US would force allies to shoulder defence costs that the US has borne for decades.

Stoltenberg acknowledged that the US was "stepping up its support" for European allies "and increasing its presence".

But he noted that defence spending also rose in European NATO members and Canada this year.

"This year we expect a further increase of three percent -- or US $8 billion (7.26 billion euros)," he said.