MOSCOW — In a major escalation of the war in Ukraine, Russian President Vladimir Putin raised the threat of a nuclear response in the conflict in Ukraine and ordered the country's reservists to mobilise, as Moscow seeks to buttress his army's flagging manpower and regain the offensive following stinging losses on the battlefield.

"Russia will use all the instruments at its disposal to counter a threat against its territorial integrity — this is not a bluff," Putin said in a national address that blamed the West for the continuing conflict in Ukraine, where he said his troops were facing the best of Western troops and weapons.