Prime Minister Manmohan Singh today said Naxalism remains the biggest internal security challenge and it is imperative to control left-wing extremism for the country's growth.

His government has not underestimated the problem of Naxalism, Singh said at a national press conference here to mark the completion of one year of UPA-II in office.

"I have been saying for the last three years that Naxalism remains the biggest internal security challenge facing our country," he said.

Asked if his government had underestimated the Naxals, he said, "We have not underestimated the problem of Naxalism."

He said there was no difference of opinion between the Central and the state governments on the issue of left-wing extremism.

"I have spoken to the Chief Ministers of the states many times on the Naxal issue. The Chief Ministers understand that it is imperative to control Naxalism for the country's growth," the Prime Minister said.

Singh said to exploit full benefit of economic reforms, it is important to control Naxalism and terrorist elements.

"If we don't, it can affect our growth," he said.

On the issue of terrorism, the Prime Minister said the government is determined to squarely tackle the threat of terrorism and ideological extremism of various kinds.

He said that terrorism is a major national security issue and it has no religion.

Terrorism, he said, is being sponsored by particular religious elements and has to be dealt with "effectively" and "purposefully".