The US has announced that it will support plans to train Ukrainian pilots on American-made F-16 fighter jets.
The move paves the way for European nations to send their own jets to Kyiv.
National security adviser Jake Sullivan said President Joe Biden "informed his G7 counterparts" of the decision at the bloc's summit in Japan on Friday.
Ukraine's President Volodymyr Zelensky, who has arrived in Hiroshima to attend the conference with Rishi Sunak and other allies, called it an historic decision, saying "peace will become closer".
The US has previously expressed concerns about escalating the conflict, but White House National Security Adviser Jake Sullivan says this change will boost Ukraine's forces.
"We have given Ukraine what it needs based on close consultations between our military and theirs," he said.
"And now we have turned to discussions about improving the Ukrainian air force as part of our long-term commitment to Ukraine's self-defense.
"So, 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."
Mr Zelensky tweeted: "Japan. G7. Important meetings with partners and friends of Ukraine. Security and enhanced co-operation for our victory. Peace will become closer today."
Mr Biden, who is attending the G7 with other members France, Italy, Germany, Japan and Canada, as well as the EU, also announced training for Ukrainian pilots.
The Prime Minister welcomed the decision from the summit, having pressed allies to provide the Ukrainian president with the jets he has been calling for.
Mr Sunak tweeted: "Welcome announcement that the US will approve the training of Ukrainian pilots on F-16 fighter jets.
"The UK will work together with the USA and the Netherlands, Belgium and Denmark to get Ukraine the combat air capability it needs.
"We stand united."
The RAF does not have any US-manufactured F-16s, which can only be exported if Washington legally approves.
Mr Zelensky is expected to take part in meetings with the G7 on Sunday.
Mr Zelensky's attendance at the G7, the group that Russia was expelled from over its 2014 annexation of Crimea, is another show of solidarity from western allies.