Ukrainian President Volodymyr Zelenskyy is preparing to visit Washington on Wednesday, sources say, in his first known trip outside the country since Russia's invasion began in February.
Three sources, including two in Congress, confirmed plans for the visit.
They said Mr Zelenskyy's visit, while expected, could still be called off at the last minute because of security concerns.
The visit to Washington is set to include an address to Congress on Capitol Hill and a meeting with President Joe Biden.
Legislators are set to vote on a year-end spending package that includes about $45 billion in emergency assistance to Ukraine.
The US is also preparing to send Patriot surface-to-air missiles to the country.
The latest tranche would be the biggest US infusion of funding yet to Ukraine, above Mr Biden’s $37 billion emergency request, and would ensure that finances flow to the war effort for months to come.
House Speaker Nancy Pelosi encouraged legislators to be present for Wednesday evening’s session.
“We are ending a very special session of the 117th Congress with legislation that makes progress for the American people as well as support for our democracy,” Ms Pelosi wrote on Tuesday in a letter to colleagues.
“Please be present for a very special focus on democracy Wednesday night.”
Mr Zelenskyy has almost daily addressed various parliaments and international organisations by video, and has sent his wife to foreign capitals to appeal for assistance.
The visit comes a day after he made a dangerous trip to what he called the hottest spot on the 1,300km front line, the city of Bakhmut in Ukraine’s contested Donetsk province.
In a video released by his office from the Bakhmut visit, Mr Zelenskyy was handed a Ukrainian flag and alluded to delivering it to US leaders.
“The guys handed over our beautiful Ukrainian flag with their signatures for us to pass on,” Mr Zelenskyy said in the video.
“We are not in an easy situation. The enemy is increasing its army. Our people are braver and need more powerful weapons.
"We will pass it on from the boys to the Congress, to the president of the United States. We are grateful for their support but it is not enough. It is a hint — it is not enough.”
The US has committed almost $20 billion in security assistance to Ukraine since Russia launched its invasion on February 24, figures from the Pentagon show.
The US is also providing intelligence to Ukrainian forces and helping Kyiv to fend off Russian cyber attacks and efforts at sabotage, American and Ukrainian officials say.
Mr Biden and Mr Zelenskyy speak regularly, as do other top officials in Washington and Kyiv.
Mr Zelenskyy has also welcomed bipartisan delegations of politicians to Ukraine and briefed them on the war effort.
He addressed Congress by video link in March. Wearing a green T-shirt with the Ukrainian flag behind him, he said the US and Ukraine shared common dreams and goals.
“Democracy, independence, freedom and care for everyone, for every person, for everyone who works diligently, who lives honestly, who respects the law,” Mr Zelenskyy said then.
“We in Ukraine want the same for our people. All that is normal part of your own life.”
His visit comes in the final days of Ms Pelosi’s role as Speaker of the House.
Republicans will take control of the House in January, while Democrats retain power in the Senate.
While Republicans set to chair key national-security committees push for continued support of Ukraine, there are growing concerns among the party's rank-and-file about the cost and duration of the effort.
News of Mr Zelenskyy's visit was first reported by Punchbowl News.