TWENTY children and six teachers have been killed after a gunman walked into a suburban US school armed with sophisticated handguns.

Connecticut State Police spokesman Lieutenant Paul Vance said 18 children were killed inside Sandy Hook Elementary School and that two more died of their wounds in hospital.

Six adults at the school were killed, including its principal, before the killer was shot - either by his own hand or police.

There were very few non-fatal injuries reported, indicating that once targeted, there was rarely any chance of escape, and that the gunman was unusually accurate in his fire.

Lieutenant Vance said the majority of killings "took place in one section of the school, in two rooms." He was reported to be carrying at least two handguns.

Local media reported that the shooter began in the kindergarten section where he killed his teacher mother and her class, then moved on. The child victims were reported to be aged between five and 10.

Initial media reports quoted police sources identifying the shooter as 24-year-old Ryan Lanza. However, after initial confusion, police now believed the killer was Lanza's brother Adam, 20, other reports said.

The surviving brother was in custody and being questioned, according to US television reports.

There are also reports that the suspect's girlfriend and another friend are missing.

In addition to the bloodbath at the school another body was found at a residence in Newtown, bringing the total bodycount so far to 28.

A tearful President Barack Obama went on national television to express his "overwhelming grief." He ordered flags to be lowered at half mast.

Of all US campus shootings, the toll was second only to the 32 murderers in 2007 massacre at Virginia Tech.

The number far exceeded the 15 killed in the 1999 Columbine High School massacre, which triggered a fierce but inconclusive debate about the United States' relaxed gun control laws.

Witnesses described a terrifying bloodbath in the small town school, shortly after classes had got under way.

"I was going back to my classroom and I heard like a person kicking on the door and I turned around I smelled smoke," an eight-year-old boy told NBC.

"Then bullets whizzed by and then a teacher pulled me into her room," he said, describing "total panic."

Other witnesses described an intense fusillade, with perhaps 100 rounds fired, and seeing a corridor splattered with blood.

"I was in the gym at the time ... we heard lots of bangs, and we thought that it was the custodian knocking stuff down. We heard screaming. And so went to the wall, and we sat down," a young boy told WCBS television.

"Then the police came in. It's like, is he in here? Then he ran out. Then somebody yelled get to a safe place, so we went to the closet in the gym and we sat there for a little while," he said, as stunned parents arrived.

"Then the police like were knocking on the door, and they're like, we're evacuating people, we're evacuating people. We ran out.

"They're police at every door leading us down this way, this way. Quick, quick, come on. We ran down to the firehouse.

There's a man that pinned down to the ground with handcuffs on," he said.

Police swarmed into the leafy neighbourhood after the shooting, while other area schools were put under lock-down, police and local media said.

A photo on The Newtown Bee's website showed officers leading more than a dozen frightened small children across a parking lot.