U.S. President Joe Biden and Britain's incoming Prime Minister Rishi Sunak decided to cooperate to help Ukraine and stand up to China.

The White House said that the two leaders also underlined the "special relationship" that exists between the U.S. and the U.K. and pledged to cooperate to enhance world security and prosperity.

They first spoke a few hours after Liz Truss, Britain's third prime minister this year, resigned after just 49 days in office, leaving Sunak to inherit an economic disaster.

Regarding the conflict started by Russia's invasion of its pro-Western neighbor in February, "the leaders agreed on the significance of working together to help Ukraine and hold Russia accountable for its aggression," based on the statement.

As reported by the White House, Biden and Sunak also decided to "address the challenges posed by China," which is currently seen as Washington's main geopolitical and economic opponent in the international arena. The two "discussed the extent of UK-US cooperation, both bilaterally and in regions such as the Indo-Pacific" as well as the more controversial matter of Northern Ireland," according to a read-out of the call that Downing Street had earlier issued.

In a tweet earlier on Tuesday, Biden congratulated Sunak. The appointment of the U.K.'s first British Asian prime minister was termed by Biden as "pretty astounding, a groundbreaking milestone".

The Ukrainian military is being armed and supported by Britain, a crucial European partner of the United States, as it attempts to fend off the Russian invasion that started in February.

As stated by a Downing Street spokesperson, Sunak has assured Volodymyr Zelenskiy, the president of Ukraine, that Britain will continue to assist Ukraine with steadfastness and "as strong as ever under his premiership." President Zelenskiy could rely on his cabinet to maintain its support, the prime minister said. Both leaders concurred that Putin's brutal administration must continue to be under pressure through ongoing economic sanctions.

Zelenskiy asserted that he thought "British leadership in defending democracy and freedom" would continue to grow. In an evening video address, he stated, "Ukraine and Britain have reached new heights in relations lately but nevertheless we still have potential to strengthen our cooperation." He also claimed that he had invited Sunak to come to Ukraine.

Prior to the conversation, Downing Street had issued a readout of the meeting in which it discussed attempts to "counter China's malign influence". At the G20 Summit in Indonesia the following month, the two presidents are anticipated to personally meet.