Two female suicide bombers killed at least 38 people in two separate attacks (8 a.m - Lubyanka station & 8:40 a.m - Park Kultury metro station) on packed Moscow metro trains on Monday, stirring fears of a broader campaign in Russia’s heartland by Islamists from the North Caucasus.

At least 72 others were injured and officials said the death toll could rise.

No group immediately claimed responsibility, but Federal Security Service (FSB) chief Alexander Bortnikov said those responsible had links to the North Caucasus where violence has intensified over the past year as Islamist militancy overlaps with clan rivalries, criminal gangs and widespread poverty.

Critics said the attacks demonstrated the failure of Kremlin policy in Chechnya, where human rights groups accuse Russian forces of brutality. Chechen rebels were blamed for that attack. Rebel leader Doku Umarov, fighting for an Islamic emirate embracing the whole region, vowed last month to take the war to Russian cities.