"You're going to run into a big buzzsaw in America if we have the NATO secretary general talking about adding Ukraine to NATO," US envoy Richard Grenell said by video link at an event on the sidelines of the World Economic Forum.

"You cannot ask the American people to expand the umbrella of NATO when the current members aren't paying their fair share, and that includes the Dutch who need to step up," Grenell said.

He was speaking after NATO secretary general and former Dutch prime minister Mark Rutte said minutes earlier that "full NATO membership is then the easiest outcome" for Ukraine if a "sustainable" peace is secured.

Grenell also echoed US President Donald Trump's call on NATO members to spend more on defence.

"We need to make sure that those leaders are spending the right amount of money. We need to be able to avoid war. And that means a credible threat from NATO," he said.

The envoy blasted Trump's predecessor for not speaking to Russian President Vladimir Putin, saying the new US president was pressuring Ukraine and Russia "to the table".

"There's a huge frustration from Americans that we're spending hundreds of billions of dollars, and our leaders aren't speaking to each other to try to solve problems," Grenell said.

The transatlantic alliance's 32 countries in 2023 set a minimum level for defence spending of two percent of gross domestic product, but Trump has suggested raising this to five percent.

Rutte acknowledged that the share had to increase.

"We have collectively to move up and we will decide on the exact number later this year, but it will be considerably more than two (percent)," Rutte said.

He also said Europe would have to pay more for continued US defence support.

"We have to be willing to do that, because at this moment, they are paying more than the Europeans. And here Trump is right," Rutte said.

After Grenell's remarks, Belgian Prime Minister Alexander De Croo reminded the audience that Putin was "the enemy", adding: "I see a lot of finger pointing between partners. And that's not helpful."