Israeli President Shimon Peres on Saturday called the massacre of 20 children and six teachers at a US school atrocious and incomprehensible and said that Israelis' hearts were with the bereaved families.
In a letter of condolence to President Barack Obama, made available to media, Peres said there was no experience that could be likened to that of parents' loss of a child.
"We stand with you today in contemplation and grief over the atrocious, incomprehensible massacre of 20 children and six adults -- educators -- at Sandy Hook Elementary School," he wrote.
"No crime is more heinous than the killing of a child," he added. "Our thoughts and prayers are with you."
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In a separate letter, Prime Minister Benjamin Netanyahu told Obama he "was shocked and horrified by the brutal massacre of innocent children and adults in an elementary school."
"In the name of the citizens of Israel I would like to convey our deep sorrow to the families of those who lost their dear ones," his office quoted him as writing.
Earlier, Tzipi Livni, head of the newly formed centrist party HaTnuah and a former foreign minister, sent a message to US Secretary of State Hillary Clinton.
"On this sad day, all Israelis share the deep sorrow of the Americans over the loss of so many lives of women, men and innocent children," she said.
Friday's shooting, in the state of Connecticut, was one of the worst school shootings in US history.