President Joe Biden has told President Volodymyr Zelensky the US will stick by Ukraine "for as long as it takes" in its war with Russia.
"You will never stand alone," Mr Biden told Mr Zelensky as he visited the White House on his first overseas trip since the Russian invasion began.
Mr Biden confirmed a new package of more than $2bn (£1.7bn) in assistance for Ukraine and promised another $45bn.
Mr Zelensky expressed his gratitude for Washington's backing.
At Wednesday's joint news conference, Mr Biden told reporters he was "not at all worried" about holding the international coalition together.
Amid concerns that some allies may be feeling the strain of the conflict's cost and disruption to global food and energy supplies, the US president said he felt "very good" about the solidarity of support for Ukraine.
Mr Biden said Russian President Vladimir Putin had "no intention of stopping this cruel war".
As Ukraine's most important ally, the US has already committed $50bn (£41bn) of humanitarian, financial and security assistance - far more than any other country.
Mr Zelensky - wearing his trademark combat-green sweatshirt and boots - expressed hope that Congress would pass the extra $45bn in aid to Ukraine to "help us to defend our values, values and independence".
Republicans - who will take control of the House of Representatives in January - have warned they will not write a "blank cheque" for Ukraine.
But Mr Zelensky, who travelled on a US Air Force jet from the Polish city of Rzeszow, said that "regardless of changes in the Congress", he believed there would be bipartisan support for his country.
After the White House meeting, the 44-year-old Ukrainian president gave an address to a joint session of Congress, where he was welcomed with a standing ovation.
He told US lawmakers his country was still standing "against all odds" and predicted "a turning point" in the conflict next year.
While vowing Ukraine would never surrender, he said it needed more weaponry.
"We have artillery, yes, thank you," he told his audience. "Is it enough? Honestly, not really."
"For the Russian army to completely pull out, more cannon and shells are needed," he added.
Concluding his speech, Mr Zelensky presented Congress with a battle flag signed by the defenders of Bakhmut, a frontline city in the east of Ukraine that he visited on the eve of his Washington trip.
The package of security assistance announced by Washington on Wednesday includes a new Patriot missile system, which is expected to help Ukraine protect its cities from missiles and drones that Russia has fired at critical facilities.