Russian President Vladimir Putin has promised retaliation after accusing Ukraine of carrying out a deadly attack on a student dormitory in an occupied part of eastern Ukraine.

Six people were killed and 39 injured in the overnight strike in the town of Starobilsk, Luhansk region, Putin said. Another 15 people were missing.

Ukraine's military said it hit the headquarters of Russia's elite Rubicon drone military unit in Starobilsk. It did not say whether it was the same building as the one identified by Russia.

"There are no military facilities, intelligence service facilities, or related services in the vicinity," Putin said in Moscow.

"Therefore, there is absolutely no basis for claiming that the munitions struck the building as a result of our air defence or electronic warfare systems," he said at a reception in his Kremlin residence on Friday.

He ordered the Russian military to prepare its "proposals" on how to retaliate.

The Russian leader said the Ukrainian strike had been carried out in three waves using 16 drones.

Russia's state-run TV showed what it said was one of the injured students, identifying her as Diana Shovkun, aged 19.

She had head injuries after being hit by a collapsing concrete slab, the TV said report said.

No photos or videos of those who Moscow says were killed were shown.

Later on Friday, Ukraine's military said its overnight strike targeted Rubicon's headquarters in Starobilsk. It accused fighters from the special drone unit of regularly striking civilians and civilian infrastructure in Ukraine.

The statement also said that Ukrainian forces "are causing damage to military infrastructure and facilities used for military purposes, strictly adhering to the norms of international humanitarian law, laws and customs of war".

On Thursday, Ukrainian President Volodymyr Zelensky said the headquarters of Russia's security service FSB was hit in the Moscow-seized area of Ukraine's southern Kherson region.

About 100 Russian "occupiers" were either killed or injured, he added.

Moscow's military has not commented on the issue. But one pro-Kremlin Telegram channel reported "casualties" after what it said was a "massive drone strike".

Ukraine has repeatedly accused Russia's military of deliberately targeting civilians since the start of Moscow's full-scale invasion in 2022 - a charge Moscow regularly denies.

Last week, Ukrainian officials said 24 people were killed - including three girls - when a Russian missile destroyed a block of a high-rising residential building in the capital Kyiv.