Ukraine's backers "will not waver", US President Joe Biden said Sunday after talks with President Volodymyr Zelensky, who is in Hiroshima on a surprise visit to the G7 summit.

Russia "will not break our resolve" to support Ukraine, Joe Biden told reporters after the end of the three-day summit that saw the Ukrainian leader steal the spotlight during a diplomatic whirlwind of talks.

"We will not waver, Putin will not break our resolve as he thought he could," Joe Biden said.

President Zelensky arrived in the Japanese city on the back of a breakthrough in his long-running campaign to win Washington's approval for allies to supply US-made fighter jets, including F-16s.

US officials had been reluctant, citing the cost and time needed for training, as well as the risk of further escalating the conflict.

But President Biden said he received a "flat assurance from Zelensky that they will not use it to go on and move onto Russian geographic territory".

"Wherever Russian troops are within Ukraine and the area, they would be able to do that," he added.

Washington also unveiled Sunday a new package of weaponry for Mr Zelensky, who relies on the West for supplies to fend off the 15-month Russian invasion.