Ukraine’s President Volodymyr Zelensky says he is on his way to Washington, where he will meet US President Joe Biden on Wednesday.
It is his first foreign trip since Russia invaded in February.
The White House has also confirmed the trip and said it will supply Ukraine with a Patriot missile battery, significantly increasing the country’s air defence capability.
Mr Zelensky will also address Congress and hold a number of meetings.
“On my way to the US to strengthen resilience and defense capabilities of Ukraine,” he wrote on Twitter.
Mr Zelensky regularly hosts foreign leaders in the capital, Kyiv, and has visited troops around Ukraine.
The Ukrainian president has also spoken frequently to world leaders over the telephone and by video call – often from his office in Kyiv.
But the surprise visit to a foreign country marks a first since the war began and also signals the importance of Ukraine’s relationship with the US, which has played a leading role in providing military support.
In its briefing ahead of Mr Zelensky’s visit, the White House confirmed a new package of nearly $2bn (£1.6bn) of security assistance for Ukraine.
That includes a new Patriot missile system, which will help Ukraine to protect its infrastructure against Russian attacks. Ukrainian officials have long been appealing for more powerful air defence systems from the West.
Russia has been targeting Ukraine’s energy sector, plunging millions into darkness in winter with temperatures several degrees below freezing.
The White House said it will train Ukrainian troops on how to use the Patriot system in a “third country” and that this “will take some time”.
Work is also currently under way in the US to push through a bill that would give Ukraine more than $40bn (£33bn) in extra funding heading into 2023.
In terms of overall spending on direct military support since the start of the conflict, the US has committed far more than any other country.
President Zelensky says the monthly cost of defence for Ukraine was about $5bn (£4.1bn).
His visit to Washington comes a day after he made an unannounced visit to the front-line city of Bakhmut, where Ukrainian and Russian forces have fought a fierce, months-long battle.
He met troops and handed out awards to soldiers, the presidency said.
The visit was a significant show of defiance – and a demonstration of support for Ukrainian forces engaged in some of the fiercest battles in recent weeks.
Soldiers gave Mr Zelensky a Ukrainian flag with their names signed on it and asked him to give it to President Biden and the US Congress, in a moment that was captured on camera.
On the same day, Russian President Vladimir Putin awarded medals inside the Kremlin to figures involved in the Russian invasion.
Since Russia invaded Ukraine on 24 February, the US military estimates that at least 100,000 Russian and 100,000 Ukrainian soldiers have been killed or injured, along with some 40,000 civilian deaths.
The UN has recorded 7.8 million people as refugees from Ukraine across Europe, including Russia. However, the figure does not include those who have been forced to flee their homes but remain in Ukraine.