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

Here is some early color ahead of Zelenskyy's scheduled 7:30 p.m. remarks to a joint meeting of Congress:

The mood is good inside with members mingling. Early attendees included incoming House Minority Leader Hakeem Jeffries, D-N.Y., and Republican Reps. Matt Gaetz of Florida and Lauren Boebert of Colorado.

Gaetz has been one of the most vocal critics in Congress of future Ukraine funding, vowing a few weeks ago not to support additional money.

Some members are wearing yellow scarves and yellow suit jackets -- an apparent reference to solidarity with Ukraine.

Shortly ahead of Zelenskyy's address, House Speaker Nancy Pelosi welcomed senators to the House chamber for the joint meeting, including Vice President Kamala Harris, president of the Senate.

Wyoming Republican Rep. Liz Cheney was seated on the Democrats' side of the House chamber next to lawmakers she served with on the Jan. 6 committee.

-ABC News' Will Steakin

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."

The training of Ukrainian troops on the Patriot missile battery could take "several months," according to a senior defense official who briefed reporters this afternoon.

"I can just say that the training will begin very soon," the official said, noting the "Ukrainians will have to complete the training in order to be able to field the system and it's the Ukrainians who are operating the system."

The Patriot missile system and JDAM kits to convert "dumb bombs" to "smart bombs" are included in the latest round of military aid to Ukraine. Ukraine has been asking for the advanced air defense system since shortly after Russia's invasion began.

The official would not provide any concrete timelines, telling reporters: "We also would obviously want to be really, really careful for operational security reasons to not you know, not be overly precise on a date, even once we have it."

-ABC News' Luis Martinez