Russian leader Vladmir Putin has called the shootings in a school in Connecticut "particularly tragic" as the majority of the 28 killed were children.
The Kremlin said in a statement:
Vladimir Putin asked Barack Obama to convey words of support and sympathy to the families and friends of the victims and expressed his empathy with the American people.
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In his first outing since the referendum result, the lead Leave campaigner has outlined his vision for a post-Brexit Britain.