At the top of their joint news conference, Zelenskyy again thanked Biden, Congress and the American people for providing defense and aide to Ukraine against Russia, which he called a "terrorist country."

"This visit to the United States became really a historic one for our relations with the United States and the American leadership," Zelenskyy said.

"I have good news returning home," he continued, raising the new nearly $2 billion aid package. "This is a very important step to create a secure airspace for Ukraine, and that's the only way we would be able to deprive the terrorist country and terror attack."

Zelenskyy said "regardless of changes in the Congress," he believes there will be support for Ukraine, showing his apparent knowledge of resistance from House Republicans.

"We need to survive this winter, we need to protect our people, and we need to be very specific in this area. This is a key humanitarian issue for us right now. This is the survival issue," he said. "We are discussing sanctions and legal pressure on the terrorist country of Russia. Russia needs to be held accountable for everything it does against us, against our people, against Europe, and the whole free world."

At a joint news conference in the White House East Room, Biden said it's important for the world to hear directly from President Zelenskyy as his country stands up to the aggression of Russia's "imperialist autocrats."

"We should be clear about what Russia is doing," Biden said. "It is purposely attacking Ukraine critical infrastructure, destroying the system to provide heat and light Ukrainian people during the coldest darkest part of the year. Russia is using winter as a weapon, Freezing people, starving people, cutting them off from one another."

Biden criticized Vladimir Putin, who he said has "no plans" to pursue peace or bring an end to the war.

Biden told Zelenskyy "you will never stand alone" and that the U.S. and its partners are ready to provide necessary assistance.

"The American people know that if we stand by in the face of such blatant attacks, on liberty and democracy and the core principles of sovereignty and territorial integrity, the world would surely face worse consequences," Biden said, adding, "The American people are prepared to have us stand up to bullies, stand up for freedom."

Zelenskyy and Biden are scheduled to hold a joint news conference in the White House East Room at 4:30 p.m. before Zelenskyy travels to Capitol Hill to address lawmakers at 7:30 p.m.

The high-security trip to Washington comes as Biden announced another $2 billion package in security assistance for Ukraine, which includes a Patriot missile defense system Zelenskyy had been requesting.

The visit also comes as Congress weighs $45 billion in funding for Ukraine, which has met resistance from some House Republicans. Asked how much of the visit is about Biden sending a message them, the White House said it's not about politics, despite the timing.

"This isn't about sending a message to a particular political party," said a senior administration official. "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 Minority Leader Kevin McCarthy -- who has previously signaled his reluctance to further back Ukrainian aid as the country fends off a Russian invasion -- told ABC News he would attend the Ukrainian president's address to Congress this evening.

"Well, by going to the Zelenskyy speech, we'd be listening to it," he said in remarks to reporters.

"We'll meet with him privately," he added.

McCarthy oversees some conservative members of the House Republican caucus who have ardently opposed additional funding for Ukraine and has himself spoken out against providing a "blank check" for the county without an audit of how it's spending the billions in U.S. aid.

Before his remarks, it was unclear if McCarthy would attend.

–ABC News' Trish Turner