The timing for when Kyiv will receive the aircraft, how many will be delivered, and which countries will provide them remains unclear, but an official stressed they will not be used for an upcoming counteroffensive against Russia.

It came as Ukrainian President Volodymyr Zelenskyy arrived in Japan for talks with G7 leaders.

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President Joe Biden has also endorsed the training of Ukrainian pilots to fly F-16s, the White House said.

Until now, he had refused Mr Zelenskyy's requests for the jets.

But speaking on the fringes of the G7 summit in Japan, White House national security adviser Jake Sullivan said Western assistance to Ukraine needed to change as the conflict changed.

"As the training unfolds in the coming months, we will work with our allies to determine when planes will be delivered, who will be delivering them, and how many," he added.

Mr Zelenskyy and Mr Biden are now set to hold face-to-face talks over the weekend at the G7 summit.

Japan's foreign ministry announced he would arrive on Sunday to meet with Prime Minister Fumio Kishida, and America's national security chief said a Zelenskyy-Biden summit was also a "safe bet".

The provision of F-16s is a long-awaited win for Mr Zelenskyy, who has regularly called on Western allies to provide powerful aircraft to his troops.

On a visit to London, he even presented Commons Speaker Sir Lindsay Hoyle with a Ukrainian air force helmet to reinforce his point.

Western leaders have argued that training on the aircraft would take too long. It would also be some of the most sophisticated hardware provided for Ukraine, raising the stakes between Russia and the West.

It is understood the F-16s may not necessarily come from the US, but the move is part of a long-term effort to strengthen Ukraine's security, a White House official said.

"Discussions about improving the Ukrainian Air Force reflect our long-term commitment to Ukraine's self-defence," the senior Biden administration official added.

Mr Biden said in an interview with ABC News in February that US military advice showed Ukraine did not need F-16s at the time.

"I'm ruling it out for now," he said.

Efforts to train Ukrainian pilots on the jets could start at sites in Europe in the coming weeks, and will take months.

Colin Kahl, the Pentagon's top policy official, previously told Congress that training Ukrainian pilots on F-16s could take "about 18 months".

Other US defence officials have said the training could be shortened to only six to nine months, based on pilots' previous training and knowledge of fighter aircraft.

Mr Kahl has previously said that it could cost up to $11bn (£9bn) to revitalise Ukraine's military with F-16 jets.

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G7 leaders have not announced who would pay to provide Ukraine with the planes.

In March, NBC News reported that two Ukrainian pilots were in the US undergoing an assessment to determine how long it could take to train them to fly attack aircraft, including F-16s.

Washington had also approved bringing up to 10 more Ukrainian pilots to the US for further assessment.