NATO chief Jens Stoltenberg on Wednesday denied the Republicans' advance in US midterm elections would undermine Western military backing for Ukraine.

Following talks with UK Prime Minister Rishi Sunak, Stoltenberg also vowed the alliance "will continue to support (Ukraine) around Kherson", after Russia ordered its troops to withdraw from the southern city.

The NATO secretary-general said he could not comment in detail on the ongoing count from Tuesday's elections for the US Congress.

"But it's absolutely clear that there is strong bipartisan support in the United States for continued support for Ukraine," he told reporters.

"That has not changed with the elections that have taken place in United States."

The US Congress committed $40 billion for Ukraine in May with support across party lines as Kyiv fights back against Russian invaders.

But Republican Kevin McCarthy, who is in line to become House of Representatives speaker, warned last month there would be no "blank cheque" for Ukraine if his party regains control.

Ukraine enjoys backing from much of the Republican base, although hard-right lawmakers close to former president Donald Trump have voiced criticism of the US support.

Stoltenberg said there were "always some voices that have a different opinion".

But opinion polls in NATO members showed that "we are ready to continue to provide support for as long as it takes", he said outside 10 Downing Street.

Prior to becoming the first foreign leader to visit the new UK prime minister, Stoltenberg observed Ukrainian troops being trained by British, Canadian and other NATO officers in southern England.

"In a dangerous world, it's even more important that North America and Europe stand together in NATO," he said, thanking Britain for its role in the alliance supporting Ukraine.

At the start of their meeting, Sunak told Stoltenberg that NATO was "a cornerstone of the UK's security", vowing together "to face the new threats that we're all seeing" from Russia and elsewhere.

Sunak is preparing painful budget cuts after inheriting an economic crisis from his short-lived predecessor Liz Truss, who had vowed to raise UK defence spending to three percent of GDP.

Stoltenberg said: "In a more dangerous world we need to invest more in our defence, and I am absolutely confident that the United Kingdom will continue to lead by example on defence spending."