Ukrainian President Volodymyr Zelensky is on his way to Washington for a summit with President Joe Biden – where he is also expected to address a joint session of Congress.
This is Mr Zelensky's first known foreign trip since Russia's invasion began in February.
The trip is taking place after 10 months of a war that has seen tens of thousands killed and wounded on both sides of the conflict, along with devastation for Ukrainian civilians.
It also comes as US politicians are set to vote on a year-end spending package that includes about 45 billion dollars (£36.9b) in emergency assistance to Ukraine and as the Pentagon prepares to send Patriot surface-to-air missiles to the country to defend itself.
Mr Zelensky headed abroad after making the dangerous trip on Tuesday to what he called the hottest spot on the 800-mile front line of the conflict, the city of Bakhmut in Ukraine's contested Donetsk province.
He praised Ukrainian troops for their "courage, resilience and strength" as artillery boomed in the background.
In a statement on Tuesday night, White House press secretary Karine Jean-Pierre said Mr Biden looks forward to the visit and that the address to Congress will demonstrate "the strong, bipartisan support for Ukraine".
"The visit will underscore the United States' steadfast commitment to supporting Ukraine for as long as it takes, including through the provision of economic, humanitarian, and military assistance," she said.
US and Ukrainian officials have made clear they do not envision an imminent resolution to the war and are preparing for fighting to continue for some time.
Mr Biden has repeated that while the US will arm and train Ukraine, American forces will not be directly engaged in the conflict.
The US and Ukrainian presidents first discussed the idea of a visit to Washington during a December phone call, said a senior US administration official, briefing reporters on condition of anonymity because of the sensitive nature of the visit.
The White House consulted with Mr Zelensky on security for his departure from Ukraine and travel to Washington, including the risk of Russian action while the Ukrainian president was out of the country, the official added, declining to detail the measures taken to safeguard the Ukrainian leader.
The official said the US expected Russia to continue its attacks on Ukrainian forces and civilian infrastructure targets despite the trip.
The portion of US funding pending before Congress would be the biggest American infusion of assistance yet to Ukraine — even more than Mr Biden's 37 billion dollars (£30.4b) emergency request — and is meant to ensure that support flows to the war effort for months to come.
On Wednesday, the US was also set to announce that it will send a major package of 1.8 billion dollars (£1.4b) in military aid to Ukraine that will for the first time include a Patriot missile battery and precision-guided bombs for its fighter jets, US officials said.