Blocking a package of support for Ukraine would be a “Christmas present” for Vladimir Putin and Xi Jinping, Lord David Cameron has warned after Senate Republicans opposed a multi-billion-dollar aid bill.

The Foreign Secretary urged allies to rally around Kyiv, describing the response to the conflict as “the great test for our generation” as he delivered a speech at the Aspen security conference in Washington DC on Thursday.

The US Congress failed to pass a 110 billion dollar (£88 billion) package of wartime funding for Ukraine and Israel as well as other national security priorities.

The White House has sounded the alarm about what might happen if further funding is not approved soon, suggesting Ukraine’s military would be stalled or overrun.

Lord Cameron used his two-day visit to the US to describe the response to Russia’s invasion as “the great test for our generation”, citing the 2008 conflict in Georgia as he warned that if Mr Putin wins “it won’t be the end of this”.

The Foreign Secretary, who was prime minister during Russia’s takeover of Crimea in 2014, warned “American lives” could be at risk if the Russian president targeted a Nato ally next.

Speaking at the conference, Lord Cameron said: “I see it as the great test for our generation, the great challenge for our generation. Are we going to defend this democracy?”

He added: “We should pass this money to the Ukrainians, we should back them and make sure that it’s Putin that loses, because if that money doesn’t get voted through, there are only two people who will be smiling.

“One of them is Vladimir Putin in Russia. The other one is Xi Jinping in Beijing. I don’t know about you, but I don’t want to give either of those people a Christmas present.”

The Foreign Secretary is expected to meet US secretary of state Antony Blinken and Republican and Democratic congressional leaders as part of the trip.

The crisis in the Middle East will also top discussions as Israel widens its offensive into southern Gaza amid growing fears about the fate of civilians.

Lord Cameron told the summit he would also raise the prospect of using frozen Russian assets to help rebuild war-torn Ukraine, a measure which Prime Minister Rishi Sunak said the UK was considering earlier this year.

He added: “I think there’s a very strong argument for taking the frozen money and spending it on rebuilding Ukraine, and that is, if you like, a down payment on the reparations that Russia will one day have to pay for the illegal invasion.”