Then US vice president Joe Biden (left) speaks to then Russian prime minister Vladimir Putin during their meeting in Moscow, Russia, 10 March 2011. (Photo: EPA / Maxim Shipenkov) 
Biden characterised Putin as "an adversary, or someone who could be an adversary", as bright and tough.
"I will make clear to President Putin that there are areas where we can cooperate if he choses," Biden said after a NATO summit in Brussels.
"If he chooses not to cooperate and acts in the way he has in the past related to cyber seurity and some other activies, we will respond, we will respond in kind," the U.S. leader said, adding he would "make clear where the red lines are" to Putin as well. (Reporting by Marine Strauss, Gabriela Baczynska Editing by John Chalmers)