US President Joe Biden "looks forward" to meeting Ukrainian leader Volodymyr Zelensky in Hiroshima on Sunday on the sidelines of a G7 summit in the Japanese city, the White House said.

A senior Biden administration official told reporters Saturday that the meeting was "scheduled for a little bit after" 2:00 pm (0500 GMT) Sunday.

"The president will continue to reiterate the United States' firm and resolute support for Ukraine going forward," the official, speaking on condition of anonymity, said.

Earlier, National Security Advisor Jake Sullivan said Biden "looks forward to the opportunity to be able to sit down face-to-face" with Zelensky.

The Ukrainian president arrived in Japan on a French government airplane Saturday after an earlier stop in Saudi Arabia.

The surprise appearance of Zelensky at the meeting of the world's richest democracies came right after the White House finally announced it would permit allied countries to supply Ukraine with American-built F16 fighter aircraft.

Ukraine has been pleading for the advanced planes to replace its aging and war-battered Soviet-era fleet, but Biden had previously said the US would not approve transfers.

Washington had expressed concern over provoking Moscow and becoming more directly involved in the war.

Sullivan said the F16 transfers would not upset the key balance Washington wants in supporting Ukraine "in a way that avoids World War III."

He said the powerful warplanes would be used only for defense.

"The United States is not enabling or supporting attacks on Russian territory," he said, "and the Ukrainians have consistently indicated that they are prepared to follow through on that."

- 'Muscle movement' -

Responding to criticism that the United States waited too long before giving allies the green light to send Ukraine their F16s, Sullivan said the Ukrainians do not need the planes for an expected offensive to drive Russians back in the coming weeks.

Instead, the F16s and accompanying training for pilots will be part of a long-term rebuilding of the Ukrainian airforce.

"We've reached a moment where it's time to look down the road and to say 'what is Ukraine going to need... to be able to deter and defend against Russian aggression?'"

"F16s, fourth-generation fighter aircraft, are part of that mix. The obvious first step there is to do the training and then to work with allies, partners and the Ukrainians to determine how to do the actual provision points as we move forward."

The senior US official would not confirm US media reports that the government will announce a new package of weapons for Ukraine on Sunday. However, he said that future shipments would likely continue to focus mostly on artillery shells, small arms ammunition and possibly HIMARS multiple rocket system ammunition.

The official said that getting Ukraine F16s and training pilots would still take time.

"We don't have the answers to all that right now. But the muscle movements are in place and the discussions are happening," he said, saying the breakthrough was "the result of an awful lot of work."

G7 countries further turned the screws on Russia during the Hiroshima summit by agreeing to tighten already sweeping economic sanctions on Moscow.

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