Three people have been killed and 35 taken to hospital following an attack by Ukraine on a factory in the city of Izhevsk - more than 1,000km (620 miles) from the border, Russian authorities say.
Of those injured ten had suffered serious injuries, the governor of Udmurtia Aleksandr Bechalov said, adding he had briefed President Vladimir Putin on the attack.
Drones reportedly targeted the Kupol Electromechanical Plant - a military factory which is said to produce Tor surface-to-air missile systems and radar stations.
The plant also specialises in the production of Osa air defence systems and has developed drones, according to Ukrainian media.
An Ukrainian official confirmed that two long-range drones operated by the Security Service of Ukraine (SBU) struck the Kupol plant from a distance of around 1,300 km (807 miles).
"Each such special operation reduces the enemy's offensive potential, disrupts military production chains and demonstrates that even deep in Russia's rear, there are no safe zones for its military infrastructure," the source said in comments reported by Ukrainian media.
A video posted on social media and verified by the BBC showed an explosion on the roof of a building, followed by a large plume of black smoke rising over a factory-type chimney.
Russia's civil aviation regulator Rosaviatsia imposed restrictions on operations at Izhevsk airport, before lifting them a few hours later.
This is second Ukrainian drone attack on the Kupol factory since November - although that strike had not resulted in any casualties.
For its part, Moscow continues to carry out attacks in Ukraine. At the weekend Russia launched a record 537 drones and missiles on various locations across the country, including Kyiv and the Western city of Lviv.
On Monday Ukraine's Volodymyr Zelensky granted the Hero of Ukraine award posthumously to an F-16 pilot, Lieutenant Colonel Maksym Ustymenko, who was killed while trying to repel the aerial attack.
On the battlefield, while Russia's advance on the Sumy region seems to have stalled, Moscow appears to be targeting the eastern Dnipropetrovsk region. Unconfirmed reports in Russian media suggested Moscow's forces took control of the first village in the region.
Two rounds of talks aimed at agreeing a ceasefire between Kyiv and Moscow have taken place at the behest of US President Donald Trump since May, but have failed to produce tangible results.
Last week, President Putin said Russia was ready to hold a new round of peace negotiations although he said that the Russian and Ukrainian peace proposals were "absolutely contradictory".
On Monday Zelensky again expressed scepticism of Putin's intentions. "Putin has already stolen practically half a year from diplomacy... on top of the entire duration of this war," the Ukrainian leader said.
"Russia is not changing its plans and is not looking for a way out of this war. On the contrary, they are preparing for new operations, including on the territory of European countries."
US senior envoy for Ukraine and Russia Keith Kellogg echoed this on Monday, when he wrote on X that Russia could not "continue to stall for time while it bombs civilian targets in Ukraine".
Moscow swiftly pushed back, saying it was not "interested in stalling anything" and thanking the US for its support.