Ukraine's Zelenskyy tells Congress: 'We are united ... the entire free world'

At the White House, Biden will have an "in depth strategic discussion" with Zelenskyy today about how to move forward on the battlefield, capabilities and training for Ukraine, sanctions and export controls, and the economic and energy sector, a senior administration official told reporters.

From the initial discussion of a possible visit to its final confirmation, this all came together in one week.

Biden and Zelesnkyy discussed this visit when they spoke over the phone on Dec. 11 and the White House extended a formal invitation on Dec. 14 for the Ukrainian president to visit Washington on Dec. 21. Zelenskyy's office formally accepted the invite on Friday, Dec. 16 and the visit was confirmed on Sunday.

"We wanted to host him for a program here at the White House that would involve an extended sit down with President Biden, a meeting with key members of President Biden's national security team and Cabinet, a opportunity to address the press and then an opportunity to go up to Capitol Hill to do a Joint Session of Congress."

The official stressed that ultimately this was Zelenskyy's decision to make to visit Washington, dismissing a question on whether Russia could take drastic actions with him out of the country, saying they've already been doing that "on a near daily basis."

–ABC News' Justin Gomez

Zelenskyy's trip to Washington comes as Biden will announce a "significant new package" of nearly $2 billion in security assistance for Ukraine, which includes a Patriot missile defense battery that Zelenskyy has been requesting, a senior administration official told ABC News.

"We will train Ukrainian forces on how to operate the Patriot missile battery in a third country. This will take some time, but Ukrainian troops will take that training back to their country to operate this battery."

The visit also comes as Congress weighs $45 billion in funding for Ukraine. When asked how much of this visit is also about Biden sending a message to the new GOP-controlled House on the need for US to keep the aid for Ukraine flowing, they said this is not about politics.

"This isn't about sending a message to a particular political party, this is about sending a message to Putin and sending a message to the world that America will be there for Ukraine for as long as it takes."

–ABC News' Justin Gomez

House Speaker Nancy Pelosi sent a letter Tuesday formally inviting Zelenskyy to address lawmakers on Capitol Hill.

"It is with immense respect and admiration for your extraordinary leadership that I extend on behalf of bipartisan Congressional leadership an invitation to address a Joint Meeting of Congress on Wednesday, December 21, 2022," Pelosi wrote.

Pelosi said Zelenskyy's "courageous, patriotic, indefatigable leadership has rallied not only your people, but the world, to join the frontlines of the fight for freedom."

"The fight for Ukraine is the fight for democracy itself," the letter read. "We look forward to hearing your inspiring message of unity, resilience and determination."

Pelosi and a small group of Democratic lawmakers met with Zelenskyy in Kyiv in May to reiterate support for Ukraine.

Zelenskyy shared he was on his way to the U.S. early Tuesday morning, describing the trip as a way to strengthen the resilience and defense capabilities of Ukraine.

"I hope you're having a good flight, Volodymyr. I'm thrilled to have you here. Much to discuss," Biden responded on Twitter.

The Ukrainian president will be greeted by Biden at the White House South Lawn at 2 p.m.

The two leaders will then hold a bilateral meeting in the Oval Office at 2:30. Zelenskyy last visited Biden at the White House in September 2021, just months before Russia's invasion began.

Zelenskyy and Biden will participate in a joint press conference in the East Room at 4:30 p.m. before Zelenskyy travels to Capitol Hill to address lawmakers.

House Speaker Nancy Pelosi said the joint meeting of Congress is expected to start at 7:30 p.m.