U.S. Vice-President Kamala Harris said calls for Ukraine to cede territory to Russia were "dangerous and unacceptable" as she met with Ukrainian President Volodymyr Zelenskyy on Thursday.

The comments from the Democratic nominee for president were thinly veiled criticism of suggestions from Republican candidate Donald Trump and his running mate, Ohio Sen. J.D. Vance, that Ukraine should quickly cut a deal to end the war.

"They are not proposals for peace," Harris said. "Instead, they are proposals for surrender."

U.S. support for Ukraine faces a partisan reckoning in this year's presidential election, with Trump criticizing the billions of dollars in military assistance the U.S. has provided, and Harris pledging to continue helping the besieged country.

Harris's meeting with Zelenskyy came shortly after the Ukrainian leader sat down with U.S. President Joe Biden, who announced billions of dollars more in missiles, drones, ammunition and other supplies.

The weapons include an additional Patriot missile defence battery and a new shipment of glide bombs that can be deployed from Western fighter jets, increasing their strike range.

Biden pledged to ensure that all approved funding is disbursed before he leaves office, and he said he plans to convene a meeting with other world leaders focused on Ukraine's defence during a visit to Germany next month.

"We stand with Ukraine, now and in the future," Biden said alongside Zelenskyy in the Oval Office.

"Russia will not prevail. Ukraine will prevail."

Deteriorating relationship with Trump

Meanwhile, Zelenskyy's tumultuous relationship with Trump continued to deteriorate this week.

Instead of meeting with Zelenskyy, Trump criticized him. As for U.S. support for Ukraine, Trump said, "We continue to give billions of dollars to a man who refuses to make a deal" to end the war. His message dovetails with Russian propaganda that claims intransigence by Kyiv — not aggression from Moscow — has prolonged the bloodshed.

It's the most politically treacherous landscape that Zelenskyy has encountered in Washington since Russia invaded nearly three years ago.

Ukrainian officials are anxious to maintain good relations with whomever becomes the next president of the United States, which is its biggest and most important provider of arms, money and other support.

But the effort risks slipping into the political blender of the presidential campaign, polarizing the discussion around a war that used to be a bipartisan cause célèbre in Washington.

Zelenskyy was expected to present Biden with a plan to push the war toward an endgame that would involve a negotiated settlement with Russia. He's trying to secure leverage before Biden leaves office — including acquiescence to fire long-range Western weapons deeper into Russia — as a hedge against the possibility that American support erodes after the election.

On Thursday, Zelenskyy found some bipartisan support as he visited Capitol Hill, where he was greeted by U.S. Senate Majority Leader Chuck Schumer and Senate Minority Leader Mitch McConnell.

U.S. Sen. Lindsey Graham, a South Carolina Republican, said Zelenskyy asked to use long-range weapons, such as British-supplied Storm Shadow missiles or U.S.-made ATACMS, for "maximum benefit to bring [Russian President Vladimir] Putin to the table" and increase Ukraine's negotiating position.

"If we don't make that fundamental choice this week, I think the outcome for Ukraine is dire," Graham said.

Advice from senators

Administration officials have been skeptical of Zelenskyy's request, believing the weapons could have limited benefits but increase the risk of escalating the conflict.

WATCH | Biden, Zelenskyy meet at White House: 

Ukraine President Zelenskyy visits White House to promote plan to win war

Ukrainian President Volodymyr Zelenskyy is at the White House to promote his victory plan. His visit comes as U.S. President Joe Biden announces billions of dollars in new assistance for Kyiv.

U.S. Sen. Richard Blumenthal, a Connecticut Democrat, said senators gave Zelenskyy advice on how to persuade Biden to loosen restrictions. 

Rep. Jim Himes, another Connecticut Democrat and the ranking member of the House Permanent Select Committee on Intelligence, said Zelenskyy wanted "more, faster."

"He was politely frustrated," Himes said, and specifically requested more Patriot missile defences as Russia escalates strikes on Ukraine's cities and energy grid before the winter.

Despite support from some Republicans on Capitol Hill, Zelenskyy faces a much more tense situation with Trump.

The latest round of sniping started on Sunday, when The New Yorker published an interview with Zelenskyy in which he criticized Vance, Trump's running mate, as "too radical" for suggesting that Ukraine needs to give up some territory to end the war.

Zelenskyy also dismissed Trump's boasts that he could quickly negotiate a solution, saying, "My feeling is that Trump doesn't really know how to stop the war even if he might think he knows how."

On the same day, Zelenskyy toured a Pennsylvania factory producing munitions for the war. He was joined by Democratic Gov. Josh Shapiro, a top surrogate for Harris, and Republicans criticized the visit as a political stunt in a political battleground state.

US. House Speaker Mike Johnson demanded that Zelenskyy fire the Ukrainian ambassador to the U.S., alleging that the tour was "designed to help Democrats and is clearly election interference." The Louisiana Republican didn't attend any of lawmakers' meetings with Zelenskyy on Thursday.

Max Bergmann, director of the Europe, Russia and Eurasia program at the Center for Strategic and International Studies, said Zelenskyy is in a "no-win situation" where he "can't even visit a U.S. weapons manufacturer to say thank you without being attacked."