A 12-year-old boy in a school uniform has blown himself up at a Pakistani army recruitment centre, killing at least 31 cadets.

Officials say the youngster apparently walked into the compound unheeded.

The brazen bombing in the northwestern town of Mardan suggests militants are regrouping.

"It seems the Taliban are still a very potent force because they continue to attack installations, even if they have been quiet for a time," said former general Talat Masood.

In a sign of how nervous the government is about security, soldiers at the gates of the military compound searched drivers before allowing them to transport coffins inside.

Militant operations in recent months have been mostly sectarian and have not focused on military targets.

The Taliban have previously launched bold attacks on the military, nuclear-armed Pakistan's most powerful institution.

Last March, two suicide bombers killed at least 45 people in the city of Lahore, including nine soldiers.

In 2009, Pakistani Taliban militants disguised as soldiers attacked the army's headquarters in Rawalpindi and later took 42 people hostage in a nearby office building.