President Biden on Thursday declared his unwavering support for Ukraine’s efforts to defeat Russian invaders during an Oval Office meeting with Ukrainian President Volodymyr Zelenskyy.

The Ukrainian leader was on a high-stakes trip to the U.S. this week to argue at the United Nations, on Capitol Hill and at the White House for continued support for Kyiv’s desperate struggle to hold off a Russian invasion force that has menaced his country since February 2022. Mr. Zelenskyy presented the White House with details of his “victory plan” in separate meetings with Mr. Biden and Vice President Kamala Harris, the Democratic presidential nominee.

He did not reveal details, but outlined a way for his nation to defeat Kremlin’s full-scale military assault by taking the fight to the Russians.

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“We stand with Ukraine now and in the future,” said Mr. Biden, who has helped organize a broad alliance of NATO and other allies in support of Ukraine, but has faced criticism for limiting what Ukrainian troops can do with sophisticated U.S. arms. “Let me be clear. Russia will not prevail in this war.”

Ms. Harris took a thinly veiled shot at former President Donald Trump, the Republican presidential nominee, criticizing those she said were willing to let Russian President Vladimir Putin keep the Ukrainian lands he has unilaterally occupied in the war as part of a negotiated end to the fighting.

“They are not proposals for peace,” Ms. Harris said. “Instead they are proposals for surrender. … The United States supports Ukraine not out of charity, but because it’s in our strategic interest.”

Ahead of the meeting, Mr. Biden announced an estimated $8 billion in new aid for the war-torn country, drawing down billions of dollars in military stockpiles from the Pentagon, which were set to expire Sept. 30. Mr. Biden also pledged additional Patriot air defense batteries and expanded training for F-16 pilots. 

Mr. Zelenskyy told Mr. Biden that he was grateful for his support over the past 17 months as the war raged, but has made clear his frustration that Washington has set limits on Ukraine’s ability to strike back inside Russia.

“Thank you for your strong support for the decision helping us protect Ukraine,” he said. “We deeply appreciate that Ukraine and America have stood side by side from the very first moment of this terrible Russian war.”

Mr. Biden was forced to order the aid package after Congress failed to include a request from him to extend the Presidential Drawdown Authority for Ukraine in the stopgap funding bill that passed Wednesday night. That put the White House in a position of having to allocate $5.5 billion in military assistance set to expire at the end of the fiscal year.

It remains to be seen if that will be enough to help Ukraine defeat Russia. So far, Mr. Biden has stopped short of granting Ukraine permission to fire long-wage weapons deeper into Russian territory, fearing it could draw the U.S. and NATO into a direct shooting war with a nuclear-armed Russia. Although Mr. Biden once opposed the idea, he has recently appeared more open as the war rages on.

For Mr. Zelenskyy, the meeting was critical to present his case to a receptive U.S. president. It’s unclear how the upcoming election will influence the U.S.’ posture toward Ukraine.

Ms. Harris has expressed support for continuing to aid Ukraine, but Mr. Trump and his running mate, Sen. J.D. Vance of Ohio, have expressed mounting skepticism of U.S. involvement in the war and the likelihood Ukraine can drive Russian forces out of the lands they hold in eastern and southern Ukraine.

Mr. Trump confirmed late Thursday he will meet with Mr. Zelenskyy on Friday before the Ukrainian leader returns home. As he has said before, Mr. Trump on Thursday insisted he could end the war quickly and accused the Biden administration of dragging the country and its allies into backing an unwinnable war.

Mr. Biden and Ms. Harris “have big blood on their hands — and there is no end in sight,” Mr. Trump said.

Mr. Zelenskyy’s trip came as his country is enduring a fresh round of strikes from Russia, many apparently targeting the country’s energy infrastructure with winter on the way.

The Associated Press reported that Ukraine’s air defenses battled an overnight Russian aerial attack on the capital Kyiv for five hours, officials said Thursday, as missiles and drones again hammered the Ukrainian power grid. A kindergarten, a gas pipe and around 20 cars were damaged in the city, the Kyiv Military Administration told the news service.

In western Ukraine, the air force said, Russia fired three Kinzhal hypersonic ballistic missiles — one of Russia’s most advanced weapons — “in the direction of” Starokostiantyniv, a city in the Khmelnytskyi region, on Thursday morning, the AP reported.

• This article was based in part on wire service reports.