Volodymyr Zelenskyy said he was en route to the United States, where he will hold a summit with President Joe Biden.
The highly sensitive visit marks the first departure of the Ukrainian president Ukraine Since the Russian invasion on February 24th.
Mr Zelensky tweeted that he was traveling to the US to “strengthen his country’s resilience and defense”.
He said he would discuss cooperation between the U.S. and Ukraine, adding: “I will also be speaking at Congress and some bilateral meetings.”
Mr Zelensky will meet with Mr Biden, the US president’s national security team and congressional leaders today, a senior US official said.
Mr. Biden’s message to Mr. Zelensky was that “we will stand with Ukraine as long as needed,” the official added.
It is understood that the US commander-in-chief will announce that he will send Patriot surface-to-air missiles to Ukraine to help the country repel the Russian invasion, and the US will train Ukrainian troops in third countries on how to use these missiles.
They will also equip their fighter jets with precision-guided bombs as part of military aid worth $1.8bn (£1.48bn), according to US officials.
Meanwhile, US politicians are due to vote on a year-end spending package that includes some $45bn (£37bn) in emergency aid to Ukraine.
The funding would be the largest US aid to Ukraine to date, surpassing President Joe Biden’s $37bn (£30bn) emergency request and would secure funding for the war in the coming months.
U.S. House of Representatives Speaker Nancy Pelosi said earlier that a congressional session would be held on Wednesday night with a “very special focus on democracy.”
Throughout the Russian invasion, Mr Zelensky addressed various parliaments and international organizations via video, and sent his wife to foreign capitals to seek support.
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Unannounced visits to frontline troops
It comes after Mr Zelensky made a surprise visit to troops in the front-line city of Bakhmut in Ukraine’s disputed Donetsk province.
Mr Zelensky inspected the scene of the fiercest fighting since Russia invaded Ukraine, praising the “courage, resilience and strength” of his troops.
Zelensky says he believes Russia must be held accountable
In a video released by his office, Mr Zelensky was presented with a Ukrainian flag and hinted at giving it to the US leader.
“These people handed over our beautiful signed Ukrainian flag, let’s pass it on,” Mr. Zelensky said.
“We are not in an easy situation. The enemy is increasing the army, our people are braver and need stronger weapons. We will pass it from the boys to Congress to the President of the United States. We appreciate their support, but It’s not enough. It’s a hint — it’s not enough.”