US President Barack Obama condemned the deadly Taliban raid on a Pakistani school Tuesday and promised that America would stand by the country in its struggle against violent extremism.

"By targeting students and teachers in this heinous attack, terrorists have once again shown their depravity," he said, after the attack on the army-run facility in Peshawar left at least 130 people dead.

"We stand with the people of Pakistan, and reiterate the commitment of the United States to support the government of Pakistan in its efforts to combat terrorism and extremism and to promote peace and stability in the region."

Most of those killed in the attack were children, officials said.

Six gunmen in government paramilitary uniforms went from classroom to classroom at the Army Public School, shooting children as young as 12.

The Tehreek-e-Taliban Pakistan (TTP), claiming responsibility, said it was in revenge for a major military offensive in the region.

Authorities regained control after more than seven hours of fighting that left the six militants dead, but explosive devices they left behind were hampering clearance efforts.

Prime Minister Nawaz Sharif described the attack as a "national tragedy unleashed by savages."

Since June, the Pakistani army has been waging an air and ground campaign against the Taliban in tribal areas along the border with Afghanistan.

Formed in 2007, the TTP groups a myriad of Islamist factions that have declared a "holy war" against the government for its strategic alliance with the United States after the September 11, 2001 attacks.

Tuesday's TTP attack was the second deadliest in the country's history after bombings targeting former Prime Minister Benazir Bhutto in October 2007 that killed 139 people in Karachi.