The US and its allies are planning to provide Ukraine with F-16 fighter jets, the White House has said.
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.
On meeting at the summit with Rishi Sunak, the prime minister slapped him on the back after they greeted each other with an embrace.
Mr Sunak said: “Good to see you. You made it.”
Asked if it was a good day for Ukraine, Mr Zelenskyy smiled, nodded and said: “I think so.”
Mr Sunak updated the Ukrainian leader on the “very positive progress” on providing the fighter jets to Ukraine, Downing Street said.
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Earlier US President Joe Biden authorised western allies to give advanced F-16 fighter jets to the embattled nation.
Mr Biden also endorsed the training of Ukrainian pilots to fly the warplanes, according to officials.
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 itself 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.
Welcoming the decision, having pressed allies to provide the Ukrainian president with the jets he has been calling for, Mr Sunak tweeted: “Ukraine, we’re not going anywhere.”
Mr Zelenskyy and Mr Biden are now set to hold face-to-face talks over the weekend at the G7 summit.
On arrival, Mr Zelensky tweeted: “Japan. G7. Important meetings with partners and friends of Ukraine. Security and enhanced cooperation for our victory. Peace will become closer today.”
The provision of F-16s is a long-awaited win for Mr Zelenskyy.
On a visit to London earlier this year, he even presented Commons Speaker Sir Lindsay Hoyle with a Ukrainian air force helmet to reinforce his point.
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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.
What is the F-16 and what arms does it carry?
The F-16 Fighting Falcon is a multi-role fighter aircraft developed for the United States Air Force.
It has an internal M61 Vulcan cannon and 11 locations for mounting weapons and other equipment.
Its payload typically consists of two 2,000lb (907kg) bombs, two AIM-9 Sidewinder short-range air-to-air missiles, two AIM-120 medium-range air-to-air missiles and two 2,400lb (1088kg) external fuel tanks.
The jet can travel at speeds up to 1,500mph and has a range of more than 2,002 miles.
It has a wingspan of 32ft 8in (9.9m) and a length of 49ft 5in (13.8m). It weighs 19,700lb (8935kg) without fuel and has a maximum takeoff weight of 37,500lb (17010kg).
There is both a single-seat and two-seat model of the aircraft.
During the Gulf War in 1991, F-16s were used to attack airfields, military production facilities, Scud missile sites and other targets.
They were also used in the NATO bombing of military infrastructure in former Yugoslavia in 1999, as well as in Iraq and Afghanistan.
They are operated by several countries other than the US, including Poland, Belgium, the Netherlands, and Denmark.
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 had said – in an interview with ABC News in February – that US military advice showed Ukraine did not need F-16s at the time.
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.
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.