US President Joe Biden and Britain's new Prime Minister Rishi Sunak agreed in talks Tuesday to work together to support Ukraine and stand up to China, the White House said.

They spoke for the first time hours after Sunak became Britain's third prime minister this year, inheriting an economic crisis after the resignation of Liz Truss after a mere 49-day tenure.

The two leaders also reaffirmed the "special relationship" that exists between the United States and Britain, and said they would work together to advance global security and prosperity, the White House said in a read-out of the conversation.

"The leaders agreed on the importance of working together to support Ukraine and hold Russia accountable for its aggression," the statement said of the war triggered by the Russian invasion of its pro-Western neighbor in February.

The White House said Biden and Sunak also agreed to "address the challenges posed by China," which Washington has identified as its top geopolitical and economic rival on the world stage today.

Downing St had earlier released its own read-out of the call in wich it said the pair "discussed the extent of UK-US cooperation, both bilaterally and in regions such as the Indo-Pacific," as well as the more contentious issue of Northern Ireland.

Earlier on Tuesday Biden had congratulated Sunak in a tweet.

On Monday Biden described the naming of Britain's first non-white prime minister as "pretty astounding, a groundbreaking milestone."

Britain has been a key European ally of the United States in arming and supporting the Ukrainian military as it tries to repel the Russian invasion, which began last February.

Sunak to face opposition in parliament

Rishi Sunak will on Wednesday face off against opposition lawmakers for the first time as British prime minister, in a likely raucous parliamentary session following weeks of political turmoil.

It comes after he took power Tuesday as the first UK leader of colour, vowing to repair the damage wrought by outgoing leader Liz Truss through her disastrous budget, which sparked economic carnage.

Pledging also to unite his fractured Conservatives, and an increasingly unimpressed country, Sunak began his tenure by re-appointing a host of ministers from his predecessor's top team.

The former finance minister retained Jeremy Hunt as chancellor of the exchequer, bidding to keep markets on side after he stabilised the situation with his initial appointment nearly two weeks ago.

He also kept Truss's foreign, defence, trade and culture ministers, among others, as well as controversially bringing back recently fired Home Secretary Suella Braverman.

The line-up "reflects a unified party and a cabinet with significant experience, ensuring that at this uncertain time there is continuity at the heart of government", a Downing Street source said.

The largely same-look cabinet could hold an inaugural meeting Wednesday before Sunak heads to the House of Commons for his first weekly "Prime Minister's Questions", when he will battle Labour leader Keir Starmer and other opposition lawmakers.

They will undoubtedly seek to capitalise on weeks of chaos at the top of government, and reiterate demands for a general election following the selection -- by Conservative MPs -- of their third leader in two months.

"The Tories have crashed the economy, with low wages, high prices and a cost-of-living crisis," Starmer said Tuesday, in a taste of the attack-lines to come.

"The public needs a fresh start and a say on Britain's future."

- 'Difficult decisions' -

Truss left office as the UK's shortest-serving premier in history, replaced by its youngest since 1812 and first Hindu leader.

Sunak, 42, triumphed in a 96-hour Tory leadership contest after rival contender Penny Mordaunt failed to secure enough nominations from Tory lawmakers and Boris Johnson dramatically aborted an audacious comeback bid.

Truss and Johnson offered their support -- though Johnson, who privately blamed his ex-minister for toppling him in July, is thought to be fuming and still harbouring hopes of an eventual Downing Street return.

Addressing the nation outside Number 10 shortly after his appointment by King Charles III, Sunak conceded the country faced "profound economic crisis".

"I will place economic stability and confidence at the heart of this government's agenda," he said, adding: "This will mean difficult decisions to come."

In an apparent bid for better domestic unity, Sunak held immediate calls with the devolved leaders of Scotland and Wales -- something Truss failed to do in her seven-week tenure.

In his first call with a foreign leader, Sunak told Ukrainian President Volodymyr Zelensky Britain would continue its "steadfast support" following Russia's invasion.

He also spoke to US President Joe Biden, who had earlier hailed the appointment of the first British-Indian prime minister as "groundbreaking".

"President Biden said that the UK remains America's closest ally, and the Prime Minister agreed on the huge strength of the relationship," Sunak's office said of their discussions.

European leaders offered their own congratulations, while Irish premier Micheal Martin reminded Sunak of their "shared responsibility" to safeguard peace in Northern Ireland following tensions under Johnson and Truss.

Sunak is unlikely to enjoy much, if any, of a political honeymoon, as he inherits an array of problems.

Markets -- and opposition parties -- are eagerly awaiting an October 31 Halloween fiscal statement from Hunt, which is likely to contain curbs on public spending to meet tens of billions of pounds in budget shortfalls.

It will be accompanied by much-scrutinised independent assessments of the government's sums.

Labour and others are expected to keep demanding a snap election -- not due until January 2025 at the latest -- given Sunak is the second prime minister in succession without a direct mandate from the electorate.

Opposition parties have no way to force one, unless dozens of Conservative MPs agree, which appears unlikely as a flurry of polls show Labour with its largest lead in decades.

Biden warns Russia against nuclear attack

US President Joe Biden warned Russia on Tuesday against using a nuclear weapon in the war with Ukraine, saying such an escalation would be a grave error.

Asked by reporters whether he thinks Russia is preparing a "dirty bomb" attack that it would then blame on Ukraine, Biden said: "Russia would be making an incredibly serious mistake were it to use a tactical nuclear weapon."

Russia has repeatedly warned that Ukraine could use a so-called dirty bomb on its own territory.

This is a conventional bomb laced with radioactive, biological or chemical materials which get disseminated in an explosion.

The US and its allies suspect Russia might itself use a dirty bomb in a "false flag" attack, possibly to justify use of conventional nuclear weapons by Moscow as it finds itself on the back foot in eastern and southern Ukraine.

Biden said, "I'm not guaranteeing that it's a false flag operation yet. We don't know."

The Pentagon said Tuesday that Russia would face repercussions for using such a device.

"If Russia were to employ nuclear weapons or a dirty bomb, there would be consequences," spokesman Brigadier General Pat Ryder told journalists, without providing specifics.