Ukrainian President Volodymyr Zelensky said on Telegram the Nova Poshta sorting office was struck in Kharkiv.

Pictures from the scene posted on President Zelensky's account showed the building with windows blown out.

Kharkiv's regional governor Oleh Syniehubov said the victims all worked for the postal company.

Writing on Telegram himself, Mr Syniehubov said the victims were aged between 19 and 42, with some suffering shrapnel wounds from the blast.

He said the private delivery company was "strictly a civilian site".

"The Russians have inflicted more terror on Kharkiv's peaceful population," he added.

President Zelensky said a rescue operation was continuing, with emergency services working at the scene.

Russia has not yet commented on the alleged strike, but has previously denied targeting civilians during its invasion of Ukraine.

Mr Syniehubov said seven people were in hospital in a "moderate condition" and seven men were in a "serious condition".

Kharkiv, which is Ukraine's second largest city, is located only 30km (19 miles) from the Russian border.

The north-eastern city was heavily bombed during the first weeks of the war in February 2022.

Earlier this month, Kharkiv Mayor Ihor Terekhov said Ukraine's first underground school would be built in the city to allow children to continue in-person education safely.

War in maps: Ukraine widening Russia defences breach
Meanwhile in the south, Ukraine has been waging a counter-offensive campaign since June.

The war-torn country aims to sever Russia's land corridor to the Crimean peninsula - which Russia illegally annexed in 2014.

But the counter-offensive has so far proven slow, bringing only limited territorial gains.

By Thomas Mackintosh
BBC News