Twenty children and six adults were killed by the gunman in the U.S.
Most of the victims were said to be between 5 and 10-years-old and were inside two classrooms in one section of the school when the mass murderer, identified as Adam Lanza, 20, struck.
The gunman was also said to have shot his teacher mother, Nancy Lanza, at her home before opening fire on the students inside the Sandy Hook Elementary School in Newtown, Connecticut.
Mr. Ki-moon in his condolence message to the Governor of Connecticut, Dan Malloy, said: “The targeting of children is heinous and unthinkable.’’
He extended his thoughts and prayers to the families of the victims and with all others traumatised by the “horrendous crime.”
Similarly, President Barack Obama, who became tearful as he addressed citizens of the country over the shooting, said he grieved first as a father about the massacre at the school.
“Our hearts are broken today,’’ he declared, while calling for “meaningful action” to prevent such shootings.
“The majority of those who died were children, beautiful little kids between the ages of 5 and 10-years-old,” Mr. Obama said.