Britain's Liz Truss will promise billions of pounds of support for Ukraine and condemn China's actions over Taiwan during her first international trip as prime minister.
Truss, who will address the United Nations General Assembly on Wednesday, hopes her trip to New York will reinvigorate the UK's so-called special relationship with the United States after ties soured over post-Brexit trade.
The prime minister will pledge to next year meet or exceed the STG2.3 billion ($A3.9 billion) of military aid spent on Ukraine in 2022, doubling down on support for Kyiv after Russia's invasion, a Downing Street spokesperson said.
She will also vow to help end Europe's dependence on Moscow for energy.
"We cannot see Russia succeed, but we also make need to make sure we're more energy independent and we're less dependent on those authoritarian regimes," Truss told the BBC on Tuesday.
"It's about economic growth, but it's also about economic security."
In a meeting with her Japanese counterpart Fumio Kishida on Tuesday, Truss committed to tackling "the strategic threat posed by China" following what she called provocations over Taiwan.
China claims Taiwan as its own territory and has never renounced the use of force to bring the island under its control. Democratically ruled Taiwan strongly objects to China's sovereignty claims.
Truss, who also met France's Emmanuel Macron, reaffirmed Britain's support of Lithuania's President Gitanas Nauseda in the Baltic nation's ongoing trade dispute with China.
The UN summit marks the first event in a busy return of British politics, put on hold during a period of national mourning for the late monarch.
Truss's government is expected to set out a new energy support package for businesses, a plan to help the National Health Service and much-promised tax cuts.
Truss, who left the UK hours after the funeral of Queen Elizabeth, will meet US President Joe Biden in New York on Wednesday.
UK Opposition foreign policy chief David Lammy said Truss must "bring the U.K. back in from the cold and begin rebuilding our country's diplomatic influence".
Ties between the UK and the US have been tested over Brexit and Truss's introduction of legislation to unilaterally change a post-Brexit trade agreement with Northern Ireland.
Truss told reporters she did not expect trade deal talks with the US to start in the short to medium term.
She is also taking a different approach to the economy than Biden, pledging tax cuts, including for corporations, and shrugging off concern policies to boost growth might widen inequality.
Biden has publicly rejected the notion tax cuts for the rich can benefit everyone.
"I am sick and tired of trickle-down economics," he said in a tweet.
"It has never worked."