US President Joe Biden has launched his first official overseas trip with a warning to Russia that it faces "robust and meaningful" consequences if it engages in "harmful activities".
Mr Biden made clear his intention to strengthen ties with US allies, following strained relations under the Trump administration.
President Biden arrived in the UK on Wednesday.
He will meet PM Boris Johnson to agree a new "Atlantic Charter".
The pact will be a modern version of the one agreed between Winston Churchill and Franklin Roosevelt in 1941, with a focus on challenges including climate change and security.