David Cameron has used his first trip to the US since his appointment as the UK’s foreign secretary to urge the Republican party to back Ukraine with more long-range weapons, saying the aid represented tremendous value for money. He said for 10% of the US defence budget nearly half of Russia’s prewar military assets had been destroyed. Urging the west be patient about the pace of Ukraine military advances, he argued no red line should be set on western aid save Nato troops directly fighting Russia. “There is nothing that will drive Russia further back and put Putin more on the back foot than actually seeing that Crimea, which is legally part of Ukraine, is properly under attack from Ukrainian forces,” Lord Cameron said. He said that would require the further supply of long-range weaponry. In a change of UK tone if not policy, he also called for the west to seize and not just freeze Russian central bank assets in the west, saying he had looked at all the arguments against making the move, including the potential chilling effect on investments in western economies or breaching past legal undertakings. “So far I have not seen anything that suggests it would be a bad idea,” he said. The seized assets should be used as a down payment for the reparations Russian will eventually have to pay for the illegal invasion of Ukraine, he said. Cameron was planning to raise the issue in talks with the US secretary of state, Antony Blinken. Speaking to an Aspen Security Forum in Washington, Cameron adopted a tone of optimism about Ukraine that has been absent in the British government since the departure of the former prime minister Boris Johnson. He urged Republicans not to be despondent about victory, saying Ukraine had taken back half the territory that Russia stole and sunk a fifth of the Russian Black Sea fleet. He said: “There are many aspects to this war that do look quite like the first world war, the deep trenches, the frozen lines, the big defensive elements and all of that but I think what you’ve seen is where we have gone further on weapons and helped, they can make an enormous difference.” US hesitancy led to Ukraine being supplied ACTMS long-range weapons for use only in October. “There has been a hesitancy over escalatory threats that has not been borne out,” Cameron said. “As long as you don’t cross the red line of Nato soldiers fighting Russia soldiers, we should do everything we can to continue to support Ukraine.” Cameron, who spent time in Congress meeting senior Republicans, said: “Putin’s invasion is the worst example of one state invading and wrecking the sovereignty of another state since the second world war.” On Wednesday the US Senate blocked a supplemental funding bill that included financial aid for Ukraine, Israel and Taiwan as well as provisions aimed at bolstering border security. Every Senate Republican opposed advancing the legislation. The 49 to 51 vote increases the likelihood that Congress will fail to approve more funding for Ukraine before the end of the year.