Ukrainian President Volodymyr Zelensky will pay his second wartime visit to Washington next week, the White House announced Friday, in a bid to solidify the support of his country's crucial backer which has shipped billions of dollars in aid to fight Russian invaders.

Zelensky will travel to the White House on Thursday for talks with President Joe Biden and also hold meetings at the US Congress, where elements of the rival Republican Party are hesitant as Biden seeks to push through a major new package for Ukraine.

The Ukrainian leader's trip to Washington will come after meetings with other world leaders at the UN General Assembly in New York.

Jake Sullivan, Biden's national security advisor, said that the trip came at a "critical time" as Ukraine wages a counteroffensive against Russia.

Biden will reaffirm "his commitment to continuing to lead the world in supporting Ukraine as it defends its independence, its sovereignty and its territorial integrity," Sullivan told reporters.

He contrasted Zelensky's trip to Russian President Vladimir Putin's recent summit with Kim Jong Un of North Korea, one of the world's most isolated and sanctioned countries, from which Moscow is seeking weapons.

But doubts have also grown over the future of US assistance as Congress approaches a September 30 deadline to approve funding just as the election season approaches.

Former president Donald Trump, the frontrunner for the Republican nomination to challenge Biden next year, has lashed out at US assistance, saying the money would be better spent at home and predicting an eventual triumph for Putin, for whom he has shown admiration.

- Building 'momentum' for aid -

But traditional Republicans including Senator Mitch McConnell, the party's Senate leader, support assistance to Ukraine.

"We have confidence that there will be bipartisan support for this. I think President Zelensky does as well, and he wants to build momentum towards that as we head to the end of the month," Sullivan said.

"Frankly, Republicans and Democrats both recognize that the United States cannot -- in its own naked self-interest, let alone the moral obligations we have -- walk away from Ukraine at this critical moment," Sullivan said.

Ukraine launched a counter-offensive against entrenched Russian positions in June but progress has been limited, spurring the political debate in the West over support for Kyiv.

The United States has provided $43 billion in security assistance as Ukraine holds off Russian incursions. 

Biden last month asked Congress for another $40 billion for Ukraine, both in emergency defense aid and economic and humanitarian assistance.

It will be Zelensky's second visit to Washington since Russia invaded Ukraine in February 2022. In December, he secretly flew to the US capital on his first international trip during the war, entering the White House in the military fatigues that have become his trademark.

Zelensky has felt increasingly confident in traveling overseas, from European allies to Saudi Arabia to Japan, where he met leaders at the Group of Seven summit in May.

Biden paid his own surprise visit to Kyiv in February, a highly unusual trip to a zone of active combat for the security-conscious White House.

Biden, who has sought to rebut criticism he is too old for the job, has started to air the campaign advertisements of his Kyiv visit, with the 80-year-old president strutting confidently in his sunglasses alongside Zelensky.

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