RIO DE JANEIRO — Keir Starmer gave his clearest signal yet that Ukraine could win authorization to strike targets in Russia with Britain’s own Storm Shadow missiles, arguing Kyiv must have “what it needs” to triumph.

On the 1,000th day since Russia’s full-scale invasion of Ukraine, the U.K. Prime Minister said allies must put Kyiv in the “strongest possible position” to win. It comes after Ukraine reportedly used long range U.S. missiles to hit Russia for the first time.

Russian President Vladimir Putin on Tuesday responded to his outgoing U.S. counterpart Joe Biden’s green-light for Ukraine to fire ATACMS ballistic missiles into Russia by upping his nuclear saber rattling.

But Starmer argued to broadcasters at the G20 summit in Rio that it is down to Putin alone to end the “suffering” he has inflicted on Ukraine.

“The quickest way to end this conflict is for Russia to cease, and that is the quickest, most simple and just way of ending this,” the prime minister told BBC News. 

So far Starmer has refused to set out whether he will give the go-ahead for long-range missiles from Britain’s own arsenal to be used by Ukraine to strike Russia. They have only been used to date within Ukraine’s internationally recognized territory, including Crimea.

But the request from Ukrainian President Volodymyr Zelenskyy for extended use of the Franco-British weapons — which typically rely on U.S. technology for targeting — is long-standing.

Now Biden has given approval for U.S. systems to strike Russian soil, Starmer’s government is being pressed on whether it will give the same authorization.

On Tuesday the British PM argued Ukraine must “must have what it needs for as long as it needs,” in what is being seen as a sign the Ukrainian President’s demands could be met. Still, Starmer refused to set out a definitive position, arguing that going into “operational matters” would have only one winner: Putin. 

Overnight, Ukraine hit a target in Russia with ATACMS ballistic missiles for the first time since Biden authorized their limited use, RBC-Ukraine reported. The Tass news agency, owned by the Russian state, claimed an ATACMS attack on the military facility caused a small fire.

Putin responded to Biden’s missile move Tuesday by revising his nuclear doctrine to declare that a conventional attack on Russian soil by another nation, if supported by a nuclear power, could be justification for a nuclear response.

That kind of nuclear escalation will have been front of mind for Ukraine’s allies while weighing up whether to authorize deeper attacks. Downing Street described the Kremlin’s new decree as the “latest example of irresponsibility” by a “depraved” Russian government.