"On behalf of all the men and women of the US Defense Department, I extend our deepest condolences to the people of Pakistan following today's ruthless attack on the army public school in Peshawar," Hagel said in a statement.
"This heartless slaughter of over 100 innocent students and teachers was an act of pure cowardice. The United States remains committed to supporting Pakistan in its efforts to combat extremism and ensure stability in the region," he said.
Nearly 140 people, mostly students, were killed in today's attack, which came as a retaliation to Pakistan military's campaign against the militants in the northern region of the country.
"As we stand firm with the Pakistani people and military in their fight against terrorists, we also honor their sacrifices, and join the victims' families and loved ones in their mourning," said Hagel.