A Ukrainian military jet has fired a weapon that hit a target in Russia for the first time, a Ukrainian military source told Sky News.
The source said that a "Russian command node" was struck on Sunday in the Belgorod region of western Russia.
Belgorod is close to the border with northeastern Ukraine.
It is currently unclear what type of ammunition was used in the attack, including whether or not it was a Western weapon.
The United States and France recently said that Ukrainian armed forces were allowed to use their weapons to strike military targets in Russia - from where Russian forces launch attacks against Ukraine.
Under new guidelines approved by President Joe Biden, US munitions can be used on Russian territory to help defend the city of Kharkiv in northeastern Ukraine.
Lord Cameron, the UK foreign secretary, was less specific, saying simply that it was up to Ukraine to decide how to use British weapons - such as Ukrainian-launched Storm Shadow cruise missiles airplanes.