MOSCOW - Russian President Vladimir Putin said on Thursday the Boston bombing proved his tough line on insurgents in the North Caucasus was justified and that Russia and the United States must step up cooperation on security.After receiving almost 2 million questions from the Baltic Sea to Russia's far east, Putin used his annual "hotline" dial-in to present the image of a man still in control a year into his third term and not afraid of criticism at home and abroad."If we truly join our efforts together, we will not allow these strikes and suffer such losses," he said in the phone-in, which critics say is looking increasingly outdated as he fields often predictable questions from loyal factory workers, airforce pilots and struggling mothers.But this time he made sure there were some critical voices in the audience, with former Finance Minister Alexei Kudrin taking him to task over economic decline. Putin shrugged off his criticism by jokingly calling him a "slacker."Looking stern and occasionally shifting forward in his chair to make a point, Putin took questions on issues ranging from pensions and roads to the ethnic Chechens suspected of carrying out the Boston Marathon bombings.He avoided criticizing the US failure to prevent the bombings despite Russian concerns about the brothers, but he took the chance to justify using heavy force against Islamist militants who oppose Russian rule in the North Caucasus."We have always said that action is needed and not declarations. Now two criminals have confirmed the correctness of our thesis," the former KGB spy said.Putin, who first asserted his authority by crushing a Chechen independence bid in a war over a decade ago, has long said the United States underestimates the security threat posed by the Islamist militants and rejected international accusations that Moscow's use of force in the region has been heavy-handed.His remarks underlined his intention to use heightened concern over security to win closer cooperation with the United States in the run-up to the Sochi Winter Olympics next February.The Olympics are a pet project for Putin and intended as a showcase of what Russia can achieve. A fatal attack on the Games would put those efforts in doubt.