"The news of the brazen murder of more than 120 innocent students in Peshawar is devastating," Kerry said.
"And as a father, I know exactly how hard it is when you send kids out of house into the world, to school or anywhere, and particularly in today's world," he said.
In the worst attack in Pakistan in years, Taliban militants stormed Army Public School on Warsak Road and went classroom-to-classroom shooting indiscriminately.
"The perpetrators must be brought to justice. And we pledge our full support to the people of Pakistan in this difficult hour and we will help them in any way that we possibly can," said Kerry.
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"Mothers and fathers send their kids to school to learn and to be safe and to dream and to find opportunity. And particularly at this military school in Pakistan, they sent their kids there with the hope and dreams of serving their country," he said.
"Well, this morning, wherever you live, wherever you are, those are our children, and this is the world's loss. This act of terror angers and shakes all people of conscience, and we condemn it in the strongest terms possible," he said.
Kerry also referred to the terrorist attack in Sydney.
"The United States, obviously, in recent memory, has come face to face with horrific violence on our own soil, and we have seen our citizens held hostage and murdered. So we know in a very personal way what our ally Australia is going through at this very moment. And we grieve with Australia and with the families of all those terrorized, injured, and killed," he said.
Kerry said the attacks in Peshawar and Sydney underscore that threats locally are also threats globally.
"That's why the United States is engaged in more places with more partners on more issues than ever before, and we are committed with all those allies and partners to standing up to extremism and to the extremists themselves," he said.