Ukrainian President Petro Poroshenko has urged the US to provide military assistance to his country to counter pro-Russian eastern separatists.
In an address to a joint session of Congress, he said ill-equipped young Ukrainian servicemen were fighting a "war for the free world" against Russian aggression.
US President Barack Obama has condemned Russia's actions in Ukraine.
But he has said that military support will only include non-lethal equipment.
Mr Poroshenko told the joint session of Congress in Washington that Ukrainian government forces needed more equipment - both lethal and non-lethal.
"Blankets, night-vision goggles are important, but one cannot win the war with blankets."
His impassioned plea for US support brought applause and several standing ovations.
More than 3,000 people have died in fighting between Ukrainian government forces and pro-Russian rebels in the eastern Donetsk and Luhansk regions since April.
Both the US and the European Union have recently imposed a new round of sanctions against Russia over its role in the Ukraine crisis.
In his address to Congress, Mr Poroshenko accused Russia of being motivated by an "imperial mindset" and "nostalgia for the Soviet Union" in its annexation of Crimea in March and by supporting the separatists in eastern Ukraine.
And he called on the US to give Ukraine a "special security and defence status which reflects the highest level of interaction with non-Nato allies" in the face of Russian aggression, which he described as a threat to "global security everywhere".
Analysis: Jon Sopel, BBC North America editor
Ukraine's President Petro Poroshenko was cheered as he was introduced by House Speaker John Boehner.
The president's speech was punctuated by repeated applause and standing ovations. US lawmakers wanted to show him that his fight was their fight.
But Mr Poroshenko, no doubt gratified by the reception, had an agenda wider than just bathing in the love of Congress. He wants action: enhanced military status, weaponry, tougher sanctions against Russia.
That might prove more difficult to obtain than the backslaps and warm handshakes.
Later, President Barack Obama announced $46m (£28.1m) in new assistance for the Ukrainian military, but stopped short of meeting Mr Poroshenko's request for lethal aid.
On Tuesday, Ukraine's MPs approved a bill granting self rule to parts of the Donetsk and Luhansk regions - a move described as "capitulation" by some lawmakers.
Meanwhile, the European Union has declined to comment on a report that Russian President Vladimir Putin allegedly threatened that he could send troops not only to Kiev but also to several EU capitals in Eastern Europe "within two days".
Mr Putin allegedly mentioned the Baltic cities of Riga, Vilnius and Tallinn, as well as Warsaw (Poland) and Bucharest (Romania). These countries are Nato military alliance members.
Germany's Sueddeutsche Zeitung says it has a leaked document showing that Mr Putin made the threat to Mr Poroshenko, and the Ukrainian leader then passed on the comments to EU Commission President Jose Manuel Barroso at the weekend in Kiev.
The document is an internal report by the EU's External Action Service, the paper says.
On 1 September Mr Putin was quoted by Italy's La Repubblica newspaper as telling European Commission head Jose Manuel Barroso that he could "take Kiev in two weeks".
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