WASHINGTON — President Zelensky paid a defiant wartime visit to Washington on Wednesday to thank American leaders and “ordinary Americans” for their support in fighting off Russia’s invasion and pledged there would be “no compromises” in trying to bring an end to the war. 

President Biden and Congress responded with billions in new assistance and a pledge to help Ukraine pursue a “just peace.”

Mr. Biden welcomed Mr. Zelensky to the Oval Office, saying America and Ukraine would continue to project a “united defense” as Russia wages a “brutal assault on Ukraine’s right to exist as a nation.” 

Mr. Zelensky, 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.”

Pressed on how Ukraine would try to bring an end to the conflict, Mr. Zelensky rejected Mr. Biden’s framing of a “just peace,” saying, “for me as a president, ‘just peace’ is no compromises.” 

He said the war would end once Ukraine’s sovereignty, freedom and territorial integrity were restored, as well as the “payback for all the damages inflicted by Russian aggression.”

“There can’t be any ‘just peace’ in the war that was imposed on us,” he added.

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. 

Mr. Zelensky’s visit was meant to reinvigorate support for his country in America 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, America announced a $1.8 billion military aid package for Ukraine, including for the first time 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, Mr. Biden said, is “trying to use winter as a weapon, but Ukrainian people continue to inspire the world.” Later, in a joint news conference, he said President Putin has “no intention of stopping this cruel war.”

The two leaders appeared to share a warm rapport, laughing at each others’ comments and patting each other on the back throughout the visit, though Mr. Zelensky made clear he will continue to press Mr. Biden and other Western leaders for ever more support.

He said that after the Patriot system was up and running, “we will send another signal to President Biden that we would like to get more Patriots.”

“We are in the war,” Mr. Zelensky added with a smile, as Mr. Biden chuckled at the direct request. “I’m sorry. I’m really sorry.”

Mr. Biden said it is “important for the American people, and for the world, to hear directly from you, Mr. President, about Ukraine’s fight, and the need to continue to stand together through 2023.”

After the White House meeting, the Ukrainian president traveled to Capitol Hill to give an address to Congress that he billed as a “speech to Americans.” Vice President Harris was planning to attend.