Updated 9pm with White House announcement of air defence aid.
Ukrainian President Volodmyr won a promise of "significant" new air defence weapons from the White House Thursday but he warned Kyiv could lose the war with Russia if Republican lawmakers cut the flow of billions of dollars in US military aid.
Zelensky, wearing his trademark olive green military-style shirt on his second wartime visit to Washington, also failed to get the coveted long-range US missiles that Ukraine has been seeking in the effort to beat back President Vladimir Putin's forces.
The Ukrainian leader faced a vastly different political landscape compared to his first visit in December 2022, when he received a hero's welcome and gave a speech to a joint session of Congress.
This time a grim-faced Zelensky met Republican and Democratic leaders locked in a bitter spending battle that could spark a US government shutdown, with a $24 billion aid package for Ukraine at risk.
The hard-right faction dominating the Republican Party is increasingly adamant that the aid spigot should be turned off, with Congress having already approved $100 billion in aid to date, including $43 billion in weaponry.
"To win, we must all stand together and work together," Zelensky said on social media, adding that he counted on "constant support" from the United States against Russia.
The Ukrainian leader arrived right after another wave of Russian missile strikes. The attacks -- hitting cities across the country -- killed at least three people in Kherson and wounded many in other areas
Democratic Senate leader Chuck Schumer, a major supporter of President Joe Biden's pro-Ukraine policies, said Zelensky had told him "if we don’t get the aid, we will lose the war."
Pentagon visit
As part of his bid to win over Washington, Zelensky also went to the Pentagon where he laid a wreath at a memorial for victims of the September 11, 2001 attacks and will visit the White House later Thursday.
White House National Security Advisor Jake Sullivan said he was confident that the deep US political divide would not stop the flow of aid to Ukraine.
"I continue to remain of the view that when all is said and done... there will be strong bipartisan support to continue funding Ukraine," Sullivan told reporters.
Biden was set to announce a major new arms package including "significant air defence capabilities to help Ukraine," Sullivan added.
But in a blow to Zelensky, he said Biden had rejected for now a request for longer-range ATACMS missiles that can strike up to 300 kilometres (190 miles) away.
Zelensky said he had "great dialogue" on Capitol Hill earlier, despite the lack of fanfare compared to his visit nine months ago.
He got a discreet welcome from the Republican Speaker of the House of Representatives, Kevin McCarthy, who is having trouble keeping a lid on internal party squabbling over US spending in Ukraine.
Some Republicans say the money could be better spent on US border security, while there are also concerns about the pace of Kyiv's counteroffensive and that corruption in Ukraine means the money will go to waste.
The doubts are being fuelled by messaging from former president and likely 2024 candidate Donald Trump, who has opposed more funding and frequently expressed admiration for Putin.
It's a trend that has also reached parts of the generally more pro-Ukraine Republicans in the Senate, where Senator Roger Marshall said Congress should not be "sending another blank check to Zelensky."
A group of six senators led by Republican J.D. Vance issued a joint letter declaring that "enough is enough" and vowing to block all future funding requests.
Earlier this week, Zelensky attended the UN General Assembly meeting in New York where he urged the world to stand firm with Ukraine against Russia's "genocide."
His warning came a day before Poland said it would no longer arm Ukraine in a mounting row over grain exports.
But on Thursday, the Polish Prime Minister said he had been "misinterpreted" and Warsaw clarified that it would fulfil existing arms supply deals.