UK Prime Minister Keir Starmer told US President Joe Biden on Friday that British support for Ukraine's war with Russia was "unwavering", in a first call hours after Britain's new leader took charge.

"The leaders reiterated their steadfast commitment to Ukraine and the prime minister underscored that the UK's support for Ukraine was unwavering," Starmer's Downing Street office said in a readout.

"The leaders discussed their shared commitment to the special relationship between the UK and US, and their aligned ambitions for greater economic growth," the statement added.

It said Starmer "looked forward to working side by side across the breadth of the relationship, including the AUKUS partnership and ensuring a free and open Indo-Pacific".

The UK, US and Australia have forged a new defence alliance in recent years dubbed AUKUS, which is seen as primarily aimed at countering China's growing military strength in the Asia-Pacific region.

The call between the British and American leaders comes just days before they will rub shoulders next week in Washington at a NATO summit.

Biden had earlier congratulated Starmer on leading his centre-left Labour party to a landslide general election victory.

"I look forward to our shared work in support of freedom and democracy around the world, and to further strengthening the special relationship between our two countries," Biden said on X.

The UK readout said the two leaders also "reflected on their shared commitment to protecting the gains of the Belfast (Good Friday) Agreement," referring to Northern Ireland's decades-old peace deal that Washington helped secure.

The accord has become strained in recent years by Britain's exit from the European Union.

That has left Northern Ireland with the UK's only land border with the EU, along the sensitive Irish border.