World leaders Friday expressed their deep shock over the shooting spree which took place earlier in the day in the US state of Connecticut, killing at least 28 people, including 20 school children.

In a letter to Connecticut Governor Dan Malloy, UN Secretary-General Ban Ki-moon voiced his "deepest condolences" over the victims, calling the rampage "shocking murders," spokesman Martin Nesirky told reporters in New York.

"The secretary-general said that the targeting of children is heinous and unthinkable, and extended his thoughts and prayers to the families of the victims and all others traumatized by this horrendous crime," said the spokesman.

European Commission President Jose Manuel Barroso said in a statement that he was deeply shocked and horrified by the tragedy.

"Young lives full of hope have been destroyed," he said. "On behalf of the European Commission and on my own behalf, I want to express my sincere condolences to the families of the victims of this terrible tragedy."

European Union foreign policy chief Catherine Ashton also voiced her shock.

"My thoughts go out to the victims, their families and the American people at this difficult time," she said in a statement.

French President Francois Hollande expressed his condolences in a letter to US President Barrack Obama.

"I was horrified by the news of the shooting today in Sandy Hook primary school in Newtown in the state of Connecticut," he said. "Please accept my condolences on behalf of the French people and on myself."

Hollande also expressed his solidarity with the families of the victims.

In a message to Obama, Queen Elizabeth II of Britain said she was "deeply shocked and saddened" by the shooting.

"I have been deeply shocked and saddened to learn of the dreadful loss of life today in Newtown, Connecticut; particularly the news that so many of the dead are children," the queen said.

She added that her husband, Prince Philip, joined her in extending their heartfelt sympathy to Obama and the American people.

"The thoughts and prayers of everyone in the United Kingdom and throughout the Commonwealth are with the families and friends of those killed and with all those who have been affected by today's events," the queen said.

British Prime Minister David Cameron said his thoughts are with the injured and those who have lost loved ones.

"It is heartbreaking to think of those who have had their children robbed from them at such a young age, when they had so much life ahead of them," he said in a statement.

According to the police in Connecticut, six adults and 18 children were pronounced dead on the scene at the Sandy Hook Elementary School in Newtown, and two more children died later in the hospital.

State police spokesman Paul Vance told reporters that the killer was also found dead at the school and that the shooting occurred at a concentrated area in two rooms.

Another death was reported at a secondary scene in Newtown, bringing the death toll to 28.

One person was injured in the shooting but survived, Vance said. Media reports identified him as the vice principal.

Earlier reports said the shooter was 24-year-old Ryan Lanza. Police later confirmed the killer was his brother Adam Lanza, 20. Ryan Lanza was reportedly in police custody.

Reports said Adam's mother, Nancy Lanza, was also among those found dead in the school, but it was not confirmed by the police.

In an emotional statement from the White House briefing room, Obama said the United States needs to take "meaningful action" to prevent more tragedies, noting his country has seen too many shootings over the years.

The shooting is the second most deadly in the US history, exceeded only by the Virginia Tech Massacre in April 2007 which claimed 33 lives.