Volodymyr Zelenskyy has said he is on his way to the US where he will have a summit with President Joe Biden.
The highly sensitive trip marks the first time the Ukrainian president has left Ukraine since Russia invaded on 24 February.
Writing on Twitter, Mr Zelenskyy said he was heading to the US to “strengthen resilience and defence capabilities” of his country.
He said he would discuss cooperation between the US and Ukraine, adding: “I will also have a speech at the Congress and a number of bilateral meetings.”
A senior US official said that Mr Zelenskyy will meet Mr Biden, the US president’s national security team and congressional leaders today.
Mr Biden’s message to Mr Zelenskyy will be that “we will stand with Ukraine as long as it takes”, the official added.
It is understood America’s commander-in-chief will announce he will send Patriot surface-to-air missiles to Ukraine to aid the country in repelling Russia’s invasion, and the US will train the Ukrainian army in a third country on how to use them.
They will also get precision-guided bombs for their fighter jets, according to US officials, as part of military aid worth $1.8bn (£1.48bn).
Meanwhile, US politicians are set to vote on a year-end spending package, including around $45bn (£37bn) in emergency assistance to Ukraine.
The funding would be the biggest American infusion of assistance yet to Ukraine, surpassing President Biden’s $37bn (£30bn) emergency request and will ensure funding flows to the war effort for months to come.
US House of Representatives Speaker Nancy Pelosi earlier said there would be a session of Congress on Wednesday evening which would have a “very special focus on democracy”.
Mr Zelenskyy has addressed various parliaments and international organisations by video throughout Russia’s invasion and has sent his wife to foreign capitals to call for support.
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A surprise visit to troops on the frontline
It comes after Mr Zelenskyy made a surprise visit to troops in the frontline city of Bakhmut in Ukraine’s contested Donetsk province.
Visiting the scene of some of the most intense combat since Russia invaded Ukraine, Mr Zelenskyy praised his troops’ “courage, resilience and strength”.
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Zelenskyy says he believes Russia must be held accountable
In a video released by his office, Mr Zelenskyy was handed a Ukrainian flag and hinted at delivering it to US leaders.
“The guys handed over our beautiful Ukrainian flag with their signatures for us to pass on,” Mr Zelenskyy said.
“We are not in an easy situation. The enemy is increasing its army, and our people are braver and need more powerful weapons. We will pass it on from the boys to the Congress, to the president of the United States. We are grateful for their support, but it is not enough. It is a hint – it is not enough.”