Russian Prime Minister Vladimir Putin said on Thursday a train bombing that killed 26 people last week showed the threat to Russia from militant rebel groups was very high. Opening a marathon annual phone-in with the Russian people, Putin said Russia was determined to "break the spine" of terrorism and called for tough and decisive action against "criminals" who attacked their own people. "The threat of terrorism remains very high," Putin said. Islamist rebels have claimed responsibility for detonating a bomb under a luxury express train as it travelled from Moscow to St Petersburg last week, derailing and wrecking carriages and killing 26 people, including some senior officials.