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.
Details of how and when he travelled are unlikely to be made public, given the security risks involved, according to BBC.
The White House confirmed it would supply Ukraine with a Patriot missile battery, significantly increasing the country's air defence capability.
"On my way to the US to strengthen resilience and defense capabilities of Ukraine," Mr Zelensky wrote on Twitter.
He also said he would give a speech to Congress and hold a number of meetings.
The visit was, unsurprisingly, kept as a secret, with official confirmation coming only hours before it was due to start.
The US has been Ukraine's most important ally in the war, committing $50bn of humanitarian, financial and security assistance - far more than any other country.
Mr Zelensky has held regular phone calls with Western leaders since the start of the war. But by hosting him at the White House, President Biden will signal that Washington is committed to supporting the country for "as long as it takes", as has been repeatedly said.
Russia said the visit would lead to an "aggravation of the conflict".
"This does not bode well for Ukraine," Kremlin spokesperson Dmitry Peskov said.
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 battery - a sophisticated air defence system that will help Ukraine to protect its cities from missiles and drones that Russia has fired at critical facilities.
The attacks, which officials here say are designed to break Ukrainian morale, have left millions without electricity and heating as temperatures across the country plummet below freezing.
President Zelensky, however, is unlikely to receive the longer-range weapons he's been calling for, to hit Russian targets away from the front lines.
In his address to Congress on Wednesday, the Ukrainian leader will probably use the infrastructure strikes to plead for more weapons. There, he will find some Republicans who have voiced criticism about the level of US support, as Congress considers approving an extra $50bn in aid.
The monthly cost of defence for Ukraine is reportedly about $5bn (£4.1bn).
The visit to Washington comes a day after President Zelensky, dressed in combat khaki, was in Bakhmut, a front-line city in eastern Ukraine that has seen some of the fiercest battles in this war.
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 on the front line.
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.
President Zelensky has vowed to take back all territory that's under occupation, including areas invaded before February. Before the visit, in his traditional evening address, he said Ukraine would do "everything possible and impossible, expected and unexpected" to get "the results that all Ukrainians expect".
Also on Wednesday, the Kremlin says Russian President Vladimir Putin will set Russia's military goals for 2023 in an "important, voluminous speech".
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.
