Photograph: Yekaterina Shtukina/AP

The Kremlin again raised the spectre of the use of nuclear weapons in the war with Ukraine as Russian forces struggled to hold a key city in the south of the country.

Dmitry Medvedev, a former Russian president who is deputy chairman of the country’s security council, said Moscow could strike against an enemy that only used conventional weapons while Vladimir Putin’s defence minster claimed nuclear “readiness” was a priority.