"We have no concept of a preemptive strike," Putin told a forum of international experts in the southern city of Sochi in response to a question from the audience.

"In such a situation, we expect to be struck by nuclear weapons, but we will not use them" first, he said.

"The aggressor will have to understand that retaliation is inevitable, that it will be destroyed and that we, as victims of aggression, as martyrs, will go to heaven.

"They will simply die because they won't even have time to repent," he said to some laughter from the audience.

Putin last year unveiled "invincible" new weapons that would render Western missile defence systems obsolete.

In 2016 he called for the country to reinforce its military nuclear potential.