US will militarily support Ukraine for 'as long as it takes': Biden

US President Joe Biden has voiced confidence that the United States would provide long-term military support to Ukraine.

"The fact of the matter is that I believe we'll have the funding necessary to support Ukraine as long as it takes," Biden told a joint news conference with British Prime Minister Rishi Sunak.

Sunak echoed the same sentiments, saying Ukraine's supporters needed to send a strong signal to Russian President Vladimir Putin that their backing for Kiev will not weaken as the war goes on.

"The more we can put in place support for Ukraine, not just in the here and now, that support that will last for a time and for years to come, I think it sends a strong signal to (Putin) that there is no point in trying to wait us out," Sunak said.

"We're not going anywhere. We will be here for as long as it takes. And hopefully that will speed up the calculation in his mind that he should withdraw his forces."

Earlier, the two leaders in their meeting at the White House focussed on deepening economic ties between the United States and the United Kingdom.

The two leaders shared laughs and more sober sentiments in the Oval Office about the close relations between prior leaders from the two countries as they previewed topics for the meeting, including the Russia-Ukraine conflict, artificial intelligence, Northern Ireland as well as joint economic and security interests, including in Asia.

The meeting comes as Western officials try to ascertain whether Russia was responsible for the destruction of the Nova Kakhovka dam, which has displaced thousands of people and caused major economic and environmental damage. Ukraine and Russia have traded blame for the dam's destruction.

"It's daunting to think if the conversations that our predecessors had in this room, when they had to speak of wars that they fought together, peace won together," Sunak told Biden.

"Again, for the first time in over half a century, we face a war on the European continent, and as we've done before the US and the UK have stood together to support Ukraine."

Sunak joked that he would not create the same imposition on Biden that World War II-era British Prime Minister Winston Churchill did by allegedly wandering around the White House residence in the middle of the night when he was a guest of President Franklin D. Roosevelt.

Sunak brought Biden two gifts, a personalised Barbour jacket and a book written by an ancestor of Biden in the 19th Century.

US-UK trade ties

Biden and Sunak last met in Hiroshima, Japan, at the Group of Seven summit last month. They also met in Belfast in April and in San Diego in March at a trilateral event marking the defence partnership of the United States, Australia and Britain.

Thursday's discussion will touch on artificial intelligence safety, Britain said on Wednesday, saying it would host the first summit on the issue this autumn to discuss how the risks of AI can be mitigated through internationally coordinated action.

Sunak hopes to strengthen trading ties between Britain and the United States, keen to show some progress after the Biden administration quashed any speedy prospect of a post-Brexit free trade agreement between the two countries.

Britain has cemented deals with individual states and is hoping to reach other such "targeted agreements".

Sunak is also expected to try to win Biden's backing for defence minister Ben Wallace's bid to become the next secretary-general of NATO.

The British leader visited the US Capitol on Wednesday, where he met with Republican and Democratic congressional leaders.

Sunak said on Wednesday a new alliance would help London and Washington protect supply chains and navigate a global economy where new powers are "manipulating global markets, withholding crucial resources and trying to establish a stranglehold over the industries that will define our future."