The big four -- House Speaker Nancy Pelosi, Senate Majority Leader Chuck Schumer, Republican Senate Leader Mitch McConnell and House Republican Leader Kevin McCarthy -- met behind closed doors with Zelenskyy ahead of his address to Congress.

Zelenskyy was spotted walking through Statuary Hall outside the House chamber with Pelosi, Schumer and McConnell, but McCarthy, the likely incoming House speaker, was not with them for that walk.

McCarthy, also needing to win over conservatives to back his bid for speaker, has signaled he could oppose more funding to Ukraine, warning that Republicans will not write a "blank check" for aid when they soon assume the majority.

Some conservative House Republicans have made it clear they would oppose any additional funding even as Congress votes on a must-pass $1.7 trillion government funding bill this week that includes $45 billion in aid to Ukraine.

-ABC News' Katherine Faulders and Lauren Peller

President Biden defended the decision to send a Patriot missile defense system to Ukraine and denied the move was escalatory.

"It's a defensive system. It's a defensive weapon system. It's not escalatory, it's defensive," Biden said. "We'd love to not have them use it. Just stop the attacks," he said, referring to Russia.

Zelenskyy delicately added that he may call on the U.S. to send another Patriot missile in the near future, prompting laughter.

"We're working on it," Biden said with a smile.

Zelenskyy said, "We are in war. I'm sorry. I'm really sorry. That is my appreciation."