Ukrainian President Volodymyr Zelensky said he was on his way to Washington, where he will meet US President Joe Biden on Wednesday.
It is his first trip abroad since the Russian invasion in February.
Details of how and when he traveled are unlikely to be released given the security risks involved.
The White House has confirmed that it will supply Ukraine with a Patriot missile system, significantly increasing the country’s air defense capability.
“En route to the United States to strengthen Ukraine’s resilience and defense capabilities,” Zelenskyy wrote on Twitter. He also said he will speak to Congress and hold some meetings.
The visit was, of course, kept secret and the official confirmation arrived a few hours before the scheduled start.
The United States was Ukraine’s main ally during the war, providing $50 billion in humanitarian, financial, and security assistance, far more than any other country. Mr. Zelensky has had regular telephone conversations with Western leaders since the start of the war. But by hosting him at the White House, President Biden will signal Washington’s determination to support the country “for as long as necessary,” as has been said several times.
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Russia said the visit would lead to “an escalation of the conflict”.
“This does not bode well for Ukraine,” Kremlin spokesman Dmitry Peskov said. In its briefing ahead of Mr. Zelenskyy’s visit, the White House confirmed a new package of nearly $2bn (£1.6bn) in security aid to Ukraine.
These include a new Patriot missile system, a sophisticated air defense system that will help Ukraine protect its cities from missiles and drones launched by Russia at critical installations.
The attacks, which officials here say are aimed at crushing the morale of Ukrainians, have left millions without power or heating as temperatures drop below freezing across the country. However, President Zelensky is unlikely to get the long-range weapons he has called for to hit Russian targets far from the front lines.
In his speech to Congress on Wednesday, the Ukrainian leader is likely to use attacks on infrastructure to push for more weapons. There he will find some Republicans who have criticized the level of US aid as Congress considers approving another $50 billion in aid.
Monthly defense costs for Ukraine hover around $5bn (£4.1bn).
