Volodymyr Zelenskyy has secured a further $US8 billion ($11.5 billion) in military assistance from the United States in a major commitment made by President Joe Biden during the Ukrainian leader's visit to Washington. 

Mr Zelenskyy met with Mr Biden and Vice-President Kamala Harris — Donald Trump's Democratic opponent in the presidential race — at the White House on Thursday, local time, where he presented his "victory plan" for Russia's ongoing invasion of Ukraine. 

Details of the plan have been kept secret but White House officials said it contained initiatives and steps Mr Zelenskyy believes are important to end the war and prevent another. 

Later on Thursday, Republican presidential candidate Trump announced he would also meet with the Ukrainian president on Friday, despite having criticised him on his campaign trail and expressed doubts about Ukraine's ability to defeat Russia.

Mr Zelenskyy has long sought a meeting with Trump. The two men spoke by phone in July but have not met in person since Trump's term as president ended in 2021. 

"President Zelenskyy has asked to meet with me, and I will be meeting with him tomorrow morning at around 9:45 in Trump Tower (in New York)," he said during a press conference. 

Asked by a reporter if Ukraine might have to give up any of its territory to reach a peace deal with Moscow — a non-starter for Kyiv — Trump did not rule it out: "We'll see what happens."

Shortly before his comments, Ms Harris pledged support for Ukraine and — in a veiled reference to Trump — said those who would have Ukraine swap land for peace with Russia were supporting "proposals of surrender".

She was speaking alongside Mr Zelenskyy at the White House, a day after Trump assailed him for not making a deal with Russia that he claims could have ended the war.

It was Ms Harris's seventh meeting with the Ukrainian president and the third this year.

Republicans upset over Pennsylvania visit

Earlier in the week Trump looked poised to reject Mr Zelenskyy's meeting request. 

Many congressional Republicans have been furious about Mr Zelenskyy's visit on Sunday to an arms factory in Scranton, Pennsylvania, Mr Biden's hometown. 

The Republican-led House Oversight Committee opened an investigation into the trip to determine whether the Biden administration used taxpayer funds to fly Mr Zelenskyy into the state to benefit Ms Harris's presidential campaign. 

House Speaker Mike Johnson, a Republican who did not meet Mr Zelenskyy at the White House, demanded he fire his ambassador to Washington for planning the Scranton trip, called it a partisan campaign designed to shore up support for the Democrats in the battleground and clear "election interference".

Trump has stood by his criticisms of Zelenskyy on the campaign trail. 

He said on Monday the Ukrainian president wanted Ms Harris to win the election, and on Wednesday he called the Eastern European nation "dead" and "demolished". 

After agreeing to meet with him, Trump repeated his claims that he could rapidly negotiate a deal between Russia and Ukraine if he defeats Ms Harris, blaming the Biden-Harris administration for allowing Russia's invasion.

Major upgrade to weapons

The newly announced US aid includes the first shipment of a precision-guided glide bomb called the Joint Standoff Weapon, with a range of up to 130 kilometres.

The medium-range missile gives Ukraine a major upgrade to the weapons it is using to strike Russian forces, allowing the Ukrainians to do it at safer distances.

The bomb, capable of striking targets with high accuracy, is to be dropped from fighter jets. It is not a long-range missile capable of hitting targets deep inside Russia, which Ukraine has been pleading the US to let it use for many months now.

Washington has refrained from giving Ukraine this permission, fearing escalation of the war and direct hostility with Russia. 

On Wednesday, Russian President Vladimir Putin invoked the use of his nuclear weapons arsenal against any country allied with Ukraine, if long-range missiles struck Russian territory.

The Kremlin made key changes to its nuclear doctrine, which spokesperson Dmitry Peskov warned would be put into effect if Western nations "participate in an attack on our country by various means, and not necessarily nuclear ones". 

Mr Biden's package will also provide Ukraine with additional Patriot air defences, unmanned aerial systems and air-to-ground munitions, as well as strengthen Ukraine's defence industrial base and support its maintenance and sustainment requirements.

The bulk of the new aid — $US5.5 billion — is to be allocated before Monday's end of the US fiscal year, when the funding authority is set to expire. Another $US2.4 billion is under the Ukraine Security Assistance Initiative, which allows the administration to buy weapons for Ukraine from companies rather than pull them from US stocks.

He ordered the Pentagon to expand training for Ukrainian F-16 pilots, including by supporting the training of an additional 18 pilots next year.

"We're making clear that we stand with Ukraine now and in the future," Mr Biden told reporters in the Oval Office.

Mr Zelenskyy thanked Mr Biden for his support and said it was important to secure Ukraine's future in the European Union and NATO, a security alliance which has stopped short of endorsing Ukranian membership.

US to coordinate efforts of 50 nations backing Ukraine

Before visiting the White House, Mr Zelenskyy also sat down with dozens of Democratic and Republican politicians at the Capitol.

"It was bipartisan, it was positive," Republican Representative Joe Wilson, a leader of the House Ukraine caucus, told a reporter of the discussions.

US Congress has approved nearly $US175 billion of aid and military assistance for Ukraine and allied nations in the two-and-a-half years since Russia began its full-scale invasion, and many legislators say they expect Washington will need to approve more money to help Kyiv within the next several months.

Mr Zelenskyy's visit was much lower-profile than previous trips to Congress. The Senate and House left Washington on Wednesday night until after the November 5 presidential election.

Senator Dick Durbin described the victory plan he presented as "pretty basic".

"And that is to make sure that the Ukrainians have the momentum going forward, and that requires not only personnel, but equipment and supplies to finish the job," Mr Durbin told reporters.

Ukraine's defence has largely received bipartisan support in the US.

To combat Russian sanctions evasion and money laundering, Washington will act to disrupt what Mr Biden called "a global cryptocurrency network, in coordination with international partners".

The US president said he would convene a leader-level meeting of the Ukraine Defense Contact Group in Germany next month to coordinate efforts of more than 50 countries supporting Ukraine.

ABC/Reuters