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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The Prime Minister said Parliament will be given a binding vote on the final deal, but said the result will not stop Britain leaving the EU.
Officials from the train drivers' union Aslef will meet managers from Southern Railway, whilst the RMT will meet separately with Acas.
Last week the organisation wrote on Twitter: "If Obama grants Manning clemency Assange will agree to US extradition".