"No quarter can be given to those who have taken upon themselves to challenge the authority of the Indian state and the fabric of our democratic polity," he said, inaugurating the Civil Services Day in New Delhi.
The Prime Minister admitted that poor development in Maoists-affected states led to Naxals insurgency in India.
"We cannot overlook the fact that many areas in which such extremism flourishes are under-developed and many of the people, mainly poor tribals, who live in these areas have not shared equitably the fruits of development.
"It is incumbent upon us to ensure that no area of our country is denied the benefits of our ambitious developmental programmes," he said.
Citing the Dantewada massacre on Apr 6, in which 76 security personnel were killed by the Naxals, Singh said that Maoists insurgency was the 'gravest internal security threat',
"I have mentioned time and again that leftwing extremism is, perhaps, the gravest internal security threat that we face. Recent events have underscored the need for urgent and considered action to root out this problem."