Ukrainian President Volodymyr Zelenskyy and U.S. President Joe Biden on Wednesday showcased their continuing resolve to work together to push back against Russia's invasion of Ukraine, as Zelenskyy traveled to the United States in his first wartime overseas trip.

Marking the 300th day since the start of the invasion in February, the Biden administration announced $1.85 billion in new security assistance including providing Ukraine with the Patriot air defense system, considered one of the most advanced of its kind, raising its total military aid to more than $21 billion.

Through his meeting with Biden at the White House and a speech before the U.S. Congress, Zelenskyy, clad in his trademark khaki green outfit, expressed his deep gratitude for the support he has received and called for further help to win in the war that he believes will shape the world's future.

Ukrainian President Volodymyr Zelenskyy addresses a joint meeting of the U.S. Congress in the House Chamber of the U.S. Capitol in Washington on Dec. 21, 2022. (Getty/Kyodo)

"Against all odds and doom-and-gloom scenarios, Ukraine didn't fall. Ukraine is alive and kicking," the 44-year-old comedian-turned-president said at a joint meeting of Congress, having emerged from the conflict as a heroic wartime leader.

"The battle is not only for life, freedom and security of Ukrainians, or any other nation which Russia attempts to conquer. This struggle will define in what world our children and grandchildren will live," he said, adding that financial aid is not "charity" but "an investment in the global security and democracy."

The whirlwind visit to the U.S. capital by Zelenskyy took place following Russia's intensified attacks on Ukraine's critical infrastructure including its energy grid, amid freezing winter temperatures.

Biden told Zelenskyy at the outset of their meeting that Ukrainians "continue to inspire the world" with their resilience and resolve to defend their future, and pledged to continue to strengthen Ukraine's ability to fight back, particularly in air defense, criticizing Russia for seeking to use "winter as a weapon."

Ukraine President Volodymyr Zelenskyy (L) talks with U.S. President Joe Biden in the Oval Office of the White House in Washington on Dec. 21, 2022, during his first known overseas trip since Russia's invasion. (Getty/Kyodo)  

A senior defense official told reporters that the Patriot missile battery will give Ukraine "a critical long-range capability to defend its airspace," with the capabilities to intercept cruise missiles, ballistic missiles and aircraft.

Russia has been warning against Washington's potential delivery of the ground-based air defense system to Ukraine, viewing the move as provocative.

Biden emphasized during a joint press conference with the Ukrainian leader that the Patriot system is "defensive" and "not escalatory."

"We should be clear about what Russia is doing. It is purposefully attacking Ukraine's critical infrastructure, destroying the systems that provide heat and light to the Ukrainian people during the coldest, darkest part of the year," he said.

"We also know that (Russian President Vladimir) Putin has no intention -- no intention of stopping this cruel war," Biden said, noting that he also feels "very good about the solidarity of support for Ukraine" from around the world continuing into next year, citing countries including Japan.

He ended the press conference by telling Zelenskyy, "You don't have to worry. We are staying with Ukraine as long as Ukraine is there."

A senior Biden administration official, meanwhile, emphasized that the support for Kyiv will remain "broad, deep and bipartisan," playing down concerns over possible tougher oversight regarding financial aid to Ukraine as Republicans take control of the House of Representatives from January following the midterm elections.

U.S. President Joe Biden (R) and Ukrainian President Volodymyr Zelenskyy hold a joint press conference at the White House in Washington on Dec. 21, 2022. (Getty/Kyodo)

According to the Center for Strategic and International Studies, the Patriot defense system is by far the most expensive single weapon system supplied to Ukraine by the United States. The total cost will probably be around $1.1 billion -- $400 million for the system and $690 million for the missiles -- experts at the U.S. think tank said.

But the experts also said the Patriot system will "not be a game changer, being limited by the area it can cover, the single unit being deployed, and the high cost of its interceptors."

The United States will train Ukrainian forces on how to operate the Patriot missile battery in "a third country" and it will take several months to ensure the troops become able to operate the system in their own country, U.S. government officials said.

Zelenskyy has been making online appearances at parliaments around the world as well as in multinational meetings to muster international support for his country. He delivered a speech virtually before the U.S. Congress in March.

The Ukrainian leader's expression of gratitude during his trip to the United States will likely serve as an "important injection of momentum and sustenance to American and allied support for the months ahead," the senior Biden administration official said.

In his in-person speech to the U.S. Congress on Wednesday, Zelenskyy also expressed his people's resolve to pursue victory despite the grueling reality they are facing.

"In two days, we will celebrate Christmas," he said, noting this year's festivities may be "candlelit," not "because it's more romantic," but because "there will be no electricity."

He also emphasized that "We do not complain...We Ukrainians will...go through our war of independence and freedom with dignity and success."

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