Ukrainian President Volodymyr Zelensky is preparing to meet with Joe Biden on a visit to Washington today which would be his first trip outside the country since Russia's invasion.
Congressional sources have confirmed that Zelensky is to travel to the United States on a highly sensitive trip and it could be called off at any moment due to security concerns.
It comes as the US has announced it will send $1.8 billion in military aid to Ukraine in a massive package that will include for the first time a Patriot missile battery and precision guided bombs for fighter jets.
Zelensky is expected to make an address to Congress on Capitol Hill as well as meeting President Biden.
House Speaker Nancy Pelosi encouraged lawmakers to be on hand 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," Pelosi wrote Tuesday in a letter to colleagues. "Please be present for a very special focus on Democracy Wednesday night."
Zelensky has - almost daily - addressed various parliaments and international organisations by video and he has sent his wife to foreign capitals to drum up assistance.
The latest planned visit comes a day after he made a daring and dangerous trip to what he called the hottest spot on 800-mile front line, the city of Bakhmut in Ukraine's contested Donetsk province.
In a video released by his office from the Bakhmut visit, Zelensky 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," Zelensky 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 last February according to figures from the Pentagon. It is also providing intelligence to Ukrainian forces and helping Ukraine fend off Russian cyberattacks and efforts at sabotage, American and Ukrainian officials have said.
Biden and Zelensky speak regularly, as do other top officials in Washington and Kyiv, while bipartisan delegations of lawmakers have discussed the war effort.
Zelensky addressed Congress by video link in March. Wearing a green T-shirt with the Ukrainian flag behind him, Zelensky argued that 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," he said at the time. "We in Ukraine want the same for our people. All that is normal part of your own life."
