"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," Stoltenberg told AFP in an interview.

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

President Biden is set to go toe-to-toe with challenger Trump on Thursday in their first televised debate ahead of the November election.

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 key military power 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 two percent of their gross domestic product on defence.

"There are never guarantees in politics, but I expect that the US will remain strong," Stoltenberg said.

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.

"Article Five should never be questioned anyway, because that's the core responsibility of NATO, and any attempt to undermine that credibility will just increase risks," Stoltenberg said.