David Cameron has led British tributes to the victims of the primary school massacre in the US that has left 27 people, including 20 children, dead.

The Prime Minister took to Twitter to say he was "devastated" by the Connecticut shootings and described the death of so many children as "truly heartbreaking".

He later issued a statement through Downing Street offering condolences to the families of the victims.

"I was shocked and deeply saddened to hear about today's horrific shooting at Sandy Hook Elementary School in Connecticut that has killed and injured so many innocent people," he said.

"My 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.

"I offer my sincere condolences to the families, to President Obama, Governor Malloy and the American people. The thoughts of the British people are with you all at this very difficult time."

Most of the victims were children aged between five and 10 years old. The killer was also found dead inside the building after turning the gun on himself.

Liberal Democrat president Tim Farron tweeted: "The senseless murder of innocent children in Connecticut shocks us all. My thoughts & prayers are with the families affected tonight."