Russian President Vladimir Putin on Sunday called the deal reached with world powers on Tehran's nuclear programme a breakthrough but warned it was the first step in a long process. "A breakthrough step has been made, but only the first on a long and difficult path," Putin said in his first comments on the deal clinched after marathon talks in Geneva. "As the result of talks... we managed to get closer to untying one of the most difficult knots in world politics," the Russian leader said, according to a Kremlin statement.