US President Donald Trump has called on fellow members to shoulder a bigger share of the NATO budget.

The US leader has also complained that the transatlantic defence alliance is more useful for Europe than it is for the United States.

However Stoltenberg remained decidedly upbeat on Tuesday, while acknowledging the differences.

"I'm absolutely confident that the NATO summit will demonstrate the transatlantic unity, that Europe and the United States stand together despite important differences on important issues like trade, the Paris (climate) agreement or the Iran nuclear deal," the NATO leader told the France 24 television channel.

"I met president Donald Trump recently in the White House and he reconfirmed his strong personal commitment to NATO and he also recognised that European allies are investing more in defence."

Trump caused dismay in Europe during his presidential campaign when he said that NATO was "obsolete" and failing to meet the challenge posed by Islamist terror groups.

"He has a strong message about the need to do more (on defence spending) and I agree with him and the European allies also agree with him," said Stoltenberg.

Stoltenberg also said NATO hopes to start accession talks with Macedonia at the summit, which will be held in Brussels on July 11 and 12, after the small Balkan nation reached a deal with Greece to be renamed the Republic of North Macedonia.