President Barack Obama said the nation was grieving for those killed during the "tragic shooting" in Newtown, Connecticut.
During his weekly radio and television address, the President said, "We have endured far too many of these tragedies in the last few years ... We have to come together and take meaningful action to prevent more tragedies like this. Regardless of the politics".
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Two pilots have been arrested on suspicion of being under the influence of alcohol as they prepared to fly from Scotland to the New York.
Two teenagers have been arrested on suspicion of drugs offences following the death of a 17-year-old youth who collapsed at Leeds Festival.
Theresa May will gather Cabinet ministers together on Wednesday to discuss her plans for Brexit.