Joe Biden has revealed the initial findings of the missile that killed two people in Poland was fired by Ukrainian forces at an incoming Russian missile and not by Russians.
The US President and Prime Minister Rishi Sunak held an emergency meeting at the Bali G20 summit after Moscow hit Ukraine with "barbaric" strikes.
Sunak attended the morning roundtable of like-minded leaders, called by the US President, which also included French President Emmanuel Macron, German Chancellor Olaf Scholz, Japan's premier Fumio Kishida and Canada's Justin Trudeau.
They pledged to offer their "full support" for Poland's investigation into the incident, with preliminary assessments from US intelligence later suggesting the missile was fired by Ukrainian forces at an incoming Russian one.
Biden was asked if it was too early to speculate of the missile by Russia.
He said: "There is preliminary information that contests that. I don't want to say that until we completely investigate. It is unlikely in the lines of the trajectory that it was fired from Russia, but we'll see."
Sunak then held his first in-person talks with Biden, in which he hailed the US as the UK's "closest ally".
The PM told Mr Biden: "I think the partnership between our countries is unique and enduring. The United States is our closest ally, biggest trading partner, closest security partner.
"And I'm glad to see that partnership in action doing good whether it's standing up to - and I agree with your words - barbaric activity by Russia in Ukraine, but also tackling climate change, stabilising the global economy."
Downing Street said afterwards that the leaders highlighted that Russia's invasion of Ukraine is "directly responsible for precipitating global economic issues".
A No 10 spokesperson said: "The leaders agreed on the national and international importance of the strong UK-US relationship, particularly given the challenging economic times the world is currently facing.
"The Prime Minister and President Biden underscored that the actions of President Putin and his regime are directly responsible for precipitating global economic issues including rising inflation. Maintaining international pressure to ensure Putin fails in Ukraine is in everyone's interest.
"The leaders agreed to work together, and with allies, to address the economic consequences of Putin's brutality. This includes action to protect the most vulnerable in our countries and around the world, and efforts to secure our long-term energy supply."
The crisis over the incident in Poland has seen leaders at the G20 tear up their carefully organised schedules and resulted in Mr Sunak's meeting with Chinese President Xi Jinping being cancelled.
Sunak had been expected to use the meeting to call for a "frank and constructive relationship" with Beijing while also raising its human rights abuses, Downing Street said.
But Sunak had been warned by critics that he could be seen as "weak" for "drifting into appeasement with China", after he appeared to soften his rhetoric to declare the country a "systemic challenge" rather than a "threat".
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