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WASHINGTON — Ukrainian President Volodymyr Zelenskyy paid a defiant wartime visit to Washington on Wednesday to thank U.S. leaders and “ordinary Americans” for their support in fighting off Russia’s invasion and to press for continued aid in the brutal months to come. President Joe Biden and Congress responded with billions in new assistance and a pledge to help Ukraine pursue a “just peace.”

Biden welcomed Zelenskyy to the Oval Office, saying the U.S. and Ukraine would continue to project a “united defense” as Russia wages a “brutal assault on Ukraine’s right to exist as a nation.” Zelenskyy, on his first known trip outside his country since Russia invaded in February, said he wanted to visit earlier and his visit now showed the “situation is under control, because of your support.”

The highly sensitive trip was taking place after 10 months of a brutal war that has seen tens of thousands of casualties on both sides and devastation for Ukrainian civilians. Zelenskyy’s visit was meant to reinvigorate support for his country in the U.S. and around the world, amid concerns that allies are growing weary of the costly war and its disruption to global food and energy supplies. Just before his arrival, the U.S. announced its largest single delivery of arms to Ukraine, including Patriot surface-to-air missiles, and Congress planned to vote on a spending package that includes about $45 billion in emergency assistance to Ukraine.

Russia, Biden said, is “trying to use winter as a weapon, but Ukrainian people continue to inspire the world,” Biden said. He told Zelenskyy, who wore a combat-green sweatshirt and boots, that “it’s an honor to be by your side.”

After the White House meeting, Zelenskyy and Biden were to hold a news conference in the East Room. The Ukrainian president was scheduled to give an address to Congress in the evening, which Vice President Kamala Harris would attend.

Zelenskyy headed abroad after making a daring and dangerous trip Tuesday to what he called the hottest spot on the 1,300-kilometer (800-mile) front line of the war, the city of Bakhmut in Ukraine’s contested Donetsk province. He praised Ukrainian troops for their “courage, resilience and strength” as artillery boomed in the background.

Poland’s private broadcaster, TVN24, said Zelenskyy crossed into Poland early Wednesday on his way to Washington. The station showed footage of what appeared to be Zelenskyy arriving at a train station and being escorted to a motorcade of American SUVs. TVN24 said the video, partially blurred for security reasons, was shot in Przemysl, a Polish border town that has been the arrival point for many refugees fleeing the war.

Officials, citing security concerns, were cagey about Zelenskyy’s travel plans, but a U.S. official confirmed that Zelenskyy arrived on a U.S. Air Force jet that landed at Joint Base Andrews, just outside the capital, from the Polish city of Rzeszow.

House Speaker Nancy Pelosi, in her invitation to Zelenskyy to speak to lawmakers, said “the fight for Ukraine is the fight for democracy itself” and that they were looking forward to “hearing your inspiring message of unity, resilience and determination.”

U.S. and Ukrainian officials have made clear they do not envision an imminent resolution to the war and are preparing for fighting to continue for some time. The latest infusion of U.S. money would be the biggest yet — and exceed Biden’s $37 billion request.

Biden has repeated that while the U.S. will arm and train Ukraine, American forces will not be directly engaged in the war.

Biden and Zelenskyy first discussed the idea of a visit during a telephone call on Dec. 11, with a formal invitation following three days later, according to a senior U.S. administration official. Zelenskyy accepted the invitation on Friday and it was confirmed on Sunday, when the White House began coordinating with Pelosi to arrange the congressional address.

The White House consulted with Zelenskyy on security, including the risk of Russian action while he was briefly out of the country, said the official, who declined to detail the measures taken to safeguard the Ukrainian leader. The official briefed reporters on the condition of anonymity because of the sensitive nature of the visit.

The $1.85 billion U.S. military aid package will include, for the first time, a Patriot missile battery and precision guided bombs for its fighter jets, U.S. officials said. It represents an expansion in the kinds of advanced weaponry intended to bolster Ukraine’s air defenses against what has been an increasing barrage of Russian missiles.

Russia’s Foreign Ministry has said the delivery of the advanced surface-to-air missile system would be considered a provocative step and that the system and any crews accompanying it would be a legitimate target for Moscow’s military.

It was unclear when the Patriot battery would arrive on the front lines in Ukraine, given that U.S. troops will have to train Ukrainian forces. The training could take several weeks and is expected to be done in Germany.

The visit comes at an important moment, with the White House bracing for greater resistance when Republicans take control of the House in January and give more scrutiny to aid for Ukraine. GOP leader Kevin McCarthy of California has said his party will not write a “blank check” for Ukraine.

Biden and Zelenskyy frequently have talked by phone, with Biden praising Ukraine for remaining steadfast against the Russians and Zelenskyy thanking the U.S. president for support.

Senate Majority Leader Chuck Schumer opened the chamber’s session by saying that passage of the aid package and confirmation of the new U.S. ambassador to Russia, Lynne M. Tracy, would send a strong signal that Americans stand “unequivocally” with Ukraine. Tracy was confirmed later on a 93-2 vote.

Schumer, a New York Democrat, said Zelenskyy arrives not only as a president but an “ambassador to freedom itself.”

The Senate’s top Republican, Kentucky Sen. Mitch McConnell, said “the most basic reasons for continuing to help Ukraine degrade and defeat the Russian invaders are cold, hard, practical American interests.” He said “defeating Russia’s aggression will help prevent further security crises in Europe.”

Russia’s invasion, which began Feb. 24, has lost momentum. The illegally annexed provinces of Donetsk, Kherson, Luhansk and Zaporizhzhia remain fiercely contested.

With the fighting in the east at a stalemate, Moscow has used missiles and drones to attack Ukraine’s power equipment, hoping to leave people without electricity as freezing weather sets in.

In a video released by his office from the Bakhmut visit, Zelenskyy was handed a signed Ukrainian flag and alluded to delivering it to U.S. leaders. “We are grateful for their support, but it is not enough,” he said. “It is a hint — it is not enough.”

Volodymyr Fesenko, head of the Kyiv-based Penta Center think tank, said Zelenskyy’s visit to the U.S. “should determine the course of the war — Zelenskyy for the first time dared to leave Ukraine and is counting on being able to maintain, and possibly even strengthen, U.S. military and economic assistance.”

Hanna Danylovych, 43, who lives in Kyiv, Ukraine’s capital, and works as a scientist, welcomed the prospect of additional military supplies, saying “there is a great desire and dream to speed up the removal of Russian evil from our land.”

Russian President Vladimir Putin on Wednesday told his country’s military leaders that Russia will achieve its stated goals in Ukraine and use the combat experience to strengthen its military. His defense minister, Sergei Shoigu, said Russia’s military must be expanded from the current 1 million to 1.5 million during the fighting in Ukraine.