US President Barack Obama vowed Sunday to use all his power to make sure that shooting tragedies like the one that left 20 small children and six adults dead in Newtown are not repeated.

"We can't accept events like this as routine," Obama told a poignant multi-faith vigil in the Connecticut town. "We as a nation are left with some hard questions. These tragedies have to end, and to end them we must change."

An impassioned Obama offered the "love and prayers of a nation" to families of the victims, saying all Americans stood by their side in mourning the tragic loss.

"I can only hope it helps for you to know that you're not alone in your grief; that our world, too, has been torn apart; that all across this land of ours, we have wept with you," Obama said.

"We've pulled our children tight, and you must know that whatever measure of comfort we can provide, we will provide. Whatever portion of sadness that we can share with you to ease this heavy load, we will gladly bear it.

"Newtown, you are not alone."

Earlier, officials formally identified Adam Lanza, 20, as the shooter who ran amok in the picture postcard town, confirming that he shot his mother several times in the head at the house they shared before going to his old school and embarking on a gruesome killing spree.

His child victims were just six and seven years old, a loss of innocence Obama blamed on an "unconscionable evil."

"In the face of indescribable violence, in the face of unconscionable evil, you've looked out for each other," Obama said.

Lanza used his mother's bushmaster .223 assault rifle to kill 26 people at the school, including 20 children aged either six or seven, before taking his own life with a handgun as police officers closed in and sirens wailed.

The president made an urgent call for Americans to do more to prevent a repeat of the countless shooting tragedies that have scarred the nation.

"Since I've been president, this is the fourth time we have come together to comfort a grieving community torn apart by mass shootings... and in between, there have been an endless series of deadly shootings across the country," he said.

"We can't tolerate this anymore. These tragedies must end. And to end them, we must change."

"I'll use whatever power this office holds to engage my fellow citizens, from law enforcement, to mental health professionals, to parents and educators, in an effort aimed at preventing more tragedies like this," he said.

Obama acknowledged that "no single law, no set of laws can eliminate evil from the world or prevent every senseless act of violence in our society," but indicated that he would seek action.