After Manchester, Putin 'ready to help UK fight terrorism'
"We firmly condemn this cynical, inhuman crime. We expect that those behind it will not escape the punishment they deserve," Putin said in a statement published by the Kremlin.
"The Russian head of state confirmed his readiness to bolster anti-terrorism cooperation with our British partners, both on a bilateral basis and in the framework of broader international efforts," it added.
At least 22 people, including some children, were killed and 59 wounded when a suicide bomber struck as thousands of fans streamed out of a concert by US singer Ariana Grande in the English city of Manchester on Monday.
British Prime Minister Theresa May said the incident was being treated as a terrorist attack, making it the deadliest militant assault in Britain since four British Muslims killed 52 people in suicide bombings on London's transport system in July 2005.
Police said the attacker died after detonating explosives shortly after 10:33 pm (2133 GMT) at Manchester Arena, which has the capacity to hold 21,000 people. Children were among the dead, police said.
Ariana Grande, 23, later said on Twitter: "Broken. From the bottom of my heart, I am so so sorry. I don't have words."
There was no immediate claim of responsibility, but US officials drew parallels to the coordinated attacks in November 2015 by Islamist militants on the Bataclan concert hall and other sites in Paris, which claimed 130 lives.
Britain is on its second-highest alert level of "severe", meaning an attack by militants is considered highly likely.