Ukraine has fired UK-supplied Storm Shadow missiles into Russia for the first time, British media reported on Wednesday, after being given the green light from London.
Several missiles were launched against at least one Russian military target, the Financial Times said, citing an unnamed Western official.
The Times said government sources had confirmed the use of the long-range missiles for the first time.
The report, quoting Russian channels, said 12 of the missiles were fired into Kursk, a border region partially held by Ukrainian forces, on Wednesday afternoon.
The Guardian said the UK had given Ukraine permission to use the missiles in Russia in response to the deployment of North Korean troops on the border.
Broadcasters Sky News and the BBC also reported the use of the missiles.
Both Prime Minister Keir Starmer's Downing Street office and the Ministry of Defence refused to confirm the reports when contacted by AFP.
Asked about the reports in parliament on Wednesday, Defence Secretary John Healey said he was "not able to go into any further operational details".
But he said he spoke to his counterpart in Kyiv, Rustem Umerov, on Tuesday, where they discussed UK support for Ukraine.
"We've seen over recent weeks significant change in the action and in the rhetoric on Ukraine, and Ukraine's action on the battlefield speaks for itself," he said.
"We, as a nation and as a government, are doubling down on our support for Ukraine and are determined to do more."
On Tuesday at the G20 summit in Rio de Janeiro, Starmer promised that Ukraine would have "what is needed for as long as it's needed" to defeat Russia.
Several Western countries, including the UK, United States and France, have supplied Ukraine with long-range missiles. Washington gave its approval for Kyiv to use them against targets inside Russia last weekend.
Overnight Monday to Tuesday, US-supplied ATACMS were used against a military site in the Russian border region of Brianksk, prompting the Kremlin to accuse US President Joe Biden of seeking to "escalate" the conflict.
Russian President Vladimir Putin has since signed a decree loosening the rules about the use of nuclear weapons.
Storm Shadows are long-range cruise missiles developed jointly by the UK and France, where they are called SCALP. They are launched from fighter jets and have a range of more than 250 kilometres.
The UK-supplied weapons have previously been used against targets in Russian-annexed Crimea.