Ukraine’s President Volodymyr Zelensky is on his way to Washington, where he will meet US President Joe Biden.
It is his first foreign trip since Russia invaded in February.
Details of his travel are unlikely to be made public, given the security risks involved, but he was seen earlier at a Polish train station.
The US has confirmed it will supply Ukraine with a Patriot missile system, 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 (£41bn) 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.
In a speech to defence chiefs on Wednesday, Russian President Vladimir Putin said his country was fighting against Nato forces in Ukraine.
“It’s well known that the military potential and capabilities of almost all Nato’s main countries is being actively used against Russia.”