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.

He said six people were killed and 39 injured after the overnight drone strike in the town of Starobilsk, Luhansk region. Another 15 people were missing.

Kremlin-installed local officials published photos of a collapsed building and rescuers searching through the rubble.

Ukraine's military said it hit the headquarters of Russia's elite Rubicon drone military unit in Starobilsk.

The BBC has not been able to independently verify what happened in Starobilsk.

At a reception in his Kremlin residence on Friday, Putin said the Ukrainian attack was carried out in three waves using 16 drones.

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

"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," Putin said.

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

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

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

Later on Friday, Ukraine's military admitted carrying out the attack. It accused Rubicon fighters 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.