Ukrainian drone strikes on a city in Russia's Tatarstan region, more than 1,000 miles (1,600 kilometres) from Ukraine, hit a residential building on Friday, Russian officials said, forcing cancellations of state holiday celebrations.
Kyiv's army said it struck the major TANECO oil refinery in Tatarstan, as well as a factory producing synthetic rubber for use in ballistic missile production in Togliatti, on the Volga river in the Samara region.
Ukraine has upped its strikes on Russia, which it says is fair retaliation for more than four years of Moscow's full-scale offensive, sending drones into Russia as far as the Urals.
The mayor of Nizhnekamsk — an industrial city of some 240,000 people by the Kama river in the east of the republic of Tatarstan — said the city had been attacked.
"A drone hit a residential building," Mayor Radmir Belyayev said on Telegram, adding that nobody had been killed but four people were wounded.
More than 20 people had been placed in a temporary accommodation camp, he said.
"For safety reasons, it has been decided to cancel all public events scheduled for today," Belyayev said.
Russia celebrates Russia Day on Friday, marking the 1991 creation of the Russian Federation after the end of the USSR.
The main festivities on Moscow's Red Square were also moved to an indoor location outside the city centre, Russian media outlet Msk1 reported.
Around a dozen drones headed for the Russian capital were shot down on Friday, said Moscow Mayor Sergei Sobyanin.
This year, Kyiv has regularly hit Russian cities, often hundreds of miles from Ukraine, with President Vladimir Putin admitting last week that Russia needed to improve its air defences.
At least three civilians were killed in Russia and Ukraine overnight, both countries said.
US-led talks to end more than four years of conflict have led nowhere in recent months, with Washington's attention diverted towards the Iran war.
Moscow's full-scale offensive against Ukraine has turned into Europe's worst conflict since World War II, with thousands of civilians and at least tens of thousands of troops killed.