The high-stakes votes on either side of the Atlantic both feature hard-right candidates who have been historically hostile to the military alliance -- and known for warm relations with its chief adversary Russia.

"It has been proven that actually when countries have to choose... between staying in NATO with the protection, the security NATO provides, or of weakening NATO, going alone, they have chosen NATO," Stoltenberg told AFP in an interview.

"We have proven very resilient because it is so obviously in our interest to stay together, and that applies for the US, it applies for Europe," he said,

Stoltenberg was talking before the first debate between incumbent Joe Biden and Donald Trump ahead of November's presidential election in military superpower the United States.

Major ally France is also days away from a snap vote that could bring a far-right government to power for the first time since World War II -- although National Rally leader Jordan Bardella has moderated his party's stance by pledging to honour France's international commitments including on NATO.

"I expect that regardless of the outcome of the US elections, the US will remain a strong NATO ally, because that is in the security interest of the United States," said Stoltenberg, whose term ends in October.

"A strong NATO is good for Europe, but also good for the United States."

Former president Trump has rocked NATO partners on the campaign by saying he would "encourage" Russia to attack members of the Western military alliance not spending enough on their own defence.

That came after the volatile former reality TV star reportedly mulled pulling Washington out of the alliance during his term in office.

"The criticism from former President Trump has not primarily been against NATO. It has been against NATO allies not spending enough, and that has now changed," Stoltenberg said.

NATO says that 23 of its 32 members are this year set to hit the alliance's benchmark of spending at least two percent of their gross domestic product on defence.

And Stoltenberg said he expects the US to "remain strong" in that respect.

He, however, cautioned that no leader should ever cast doubt on NATO's Article Five mutual defence commitment that means an attack on one member is considered an attack on all.

"That's the core responsibility of NATO and any attempt to undermine that credibility will just increase risks," Stoltenberg said.

NATO is currently gearing up for a summit in Washington in July -- and guaranteeing embattled Ukraine can keep up its fight against Russia will be a key issue.

Stoltenberg has called on NATO allies to pledge to keep on supplying Ukraine with around 40 billion euros ($43 billion) in military support a year -- roughly equivalent to what they have been giving since Russia's invasion in 2022.

Some members have baulked at making any firm promises but Stoltenberg said he was confident NATo would agree "a pledge to support Ukraine for the long haul".

- 'Prevent war' -

NATO countries are also haggling over new wording for a summit declaration on Ukraine's push to join the alliance, with the United States and Germany refusing to lay out a clear timeframe for Kyiv's membership.

Stoltenberg said that he expects "the message, the language to be even stronger" than at a summit last year when NATO said it could invite Ukraine to join once "allies agree and conditions are met".

On the battlefield in Ukraine, Stoltenberg said that Russian forces were not capable of making major advances despite being on the front foot, and Kyiv was inflicting heavy losses with its deep strikes.

"We don't have any other indications or reason to believe that Russia has the capabilities, the strength to make big breakthroughs," he said.

The former Norwegian prime minister has led the alliance through its most consequential decade since the Cold War and will be replaced by Dutch premier Mark Rutte.

Asked if his successor could face a full-scale war between NATO and Russia during his tenure, the outgoing NATO chief said he believed the alliance's military might would keep Moscow at bay.

"I'm confident that NATO will continue to deter any armed attack against any NATO ally, as we have done for 75 years, (also) during... the most dangerous period of the Cold War," he said.

"The purpose of NATO is not to fight a war, but to prevent war and we don't see any imminent military threat against any NATO ally, and I'm absolutely certain NATO has the strength to prevent attacks also in the future."