The foreign secretary will use his visit to urge the allies to go “further and faster” in their support for the government in Kyiv, arguing that, with the right equipment, the Ukrainians can prevail against Russia.
His intervention comes after Prime Minister Rishi Sunak announced at the weekend that the UK is to become the first nation to supply Ukraine with modern western heavy tanks – a main demand of President Volodymyr Zelenskyy.
Britain will send 14 Challenger 2 main battle tanks, to be followed by about 30 AS90 self-propelled guns – a decision which, the Ukrainians hope, will unlock similar offers of support from other Nato allies.
Drones and weapons supplied to Ukraine - in pictures
“The UK, US and Canada always have each other’s backs when it counts, protecting the rules-based order for nearly 80 years," Mr Cleverly said in a statement before he went to the US.
“Today we stand united against (Vladimir) Putin’s illegal war, and we will continue to use our uniquely strong defence and security ties to ensure that, in the end, the Ukrainian people will win.”
On Tuesday, the UK government also announced that it is sending more medical supplies to Ukraine.
More than 70 pallets of bandages, crutches, wheelchairs, and other items will be donated to UK-Med, a frontline medical NGO.
The donation also includes tents to be used for temporary treatment centres, about 80 beds, more than 30,000 gowns, aprons, bandages, needles, and other medical equipment.
Russia and Ukraine conflict latest - in pictures
“Putin’s brutal bombardment of civilians and infrastructure across Ukraine is continuing to have a massive human cost," said Mr Cleverly.
“This latest UK donation will help ensure medics on the frontline can continue to provide life-saving medical support to those caught in the conflict.”
The UK has previously donated equipment and pharmaceutical supplies to UK-Med, in addition to £300,000 ($366,000) funding, to set up mobile health clinics and help train Ukrainian doctors, nurses and paramedics on how to deal with mass casualties.
This latest donation builds on that work, to support the most vulnerable civilians remaining in Ukraine.