A heavily armed young gunman slaughtered 20 small children and six adults at a school in an idyllic Connecticut town on Friday, in one of the worst mass shootings in US history.

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

Six adults at the school were killed, Vance said, before the killer was shot -- either by his own hand or police. US media said the school principal was among those killed.

Hours after the massacre, there was still no clue to the motives for the shootings in Newtown, a wooded and picturesque small town north-east of New York City.

"Evil visited this community today," State Governor Dan Malloy said.

President Barack Obama, wiping away tears and struggling to maintain his composure, said he was aghast over the tragedy.

There were almost no non-fatal injuries, indicating that once targeted, there was rarely any chance of escape, and that the gunman was unusually accurate or methodical in his fire.

Vance said just one person suffered an injury and survived.

The majority of killings, which began at around 09:30 a.m., "took place in one section of the school, in two rooms," Vance added. The children were aged between five and ten, officials said.

NBC television reported that the dead killer was Adam Lanza, 20, and that police had earlier confused him with his brother 24-year-old Ryan Lanza, whose identity card he had been carrying with him when he went into the school.

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

Another source of confusion in the police probe was whether the shooter killed his mother at the school, where she reportedly worked, or whether she was the extra victim found in a home in Newtown -- the 28th body in the day's bloodshed.

CNN and other news outlets said the shooter may have first killed his mother, before driving to her school.

Police said they expected to be able to make public identities of the victims and the killer only on Saturday.

Vance described a "massive investigation" and said that law enforcement agents working at the scene were having a hard time coping. "Between our personal experience, we've never seen anything like this. It's heart wrenching for us," he said.

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

And there were similar statements of grief and shock around the world.

The head of the European Commission Jose Manuel Barroso spoke of his "deep shock and horror," Britain's Queen Elizabeth II sent a message to Obama in which she said she was "deeply shocked and saddened."

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

The latest 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.

However, the White House on Friday scotched any suggestion that the politically explosive subject would be quickly reopened.

Witnesses described an intense fusillade fired at the elementary school, possibly numbering some 100 rounds, and seeing a corridor splattered with blood.

"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."

Police swarmed into the leafy neighborhood after the shooting, while other area schools were put under lock-down.

"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," another 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.

The police response was credited with ensuring the speedy evacuation of the rest of the school, once the gunman unleashed his attack.

"Their training kicked in. They saved a lot of lives," Vance said.

Deadly shootings are a frequent occurrence in US public places, often ending only when the gunman is shot or kills himself.

On Tuesday, a man with a semi-automatic rifle raked an Oregon shopping mall, killing two people, then taking his own life.

In the most notorious recent incident until now a 24-year-old, James Holmes, allegedly killed 12 people and wounded 58 others when he opened fire in a midnight screening of the latest Batman movie in Aurora, Colorado, in July.

However, despite the tragedies, support for tougher gun ownership laws is mixed, with many Americans opposing restrictions on what they consider to be a constitutional right to keep powerful firearms at home.