President Obama has said that John Kerry, who has been nominated as Secretary of State to replace Hillary Clinton, has "earned the respect" of leaders around the world and has played a central role in foreign policy.
President Obama said that Kerry had been "exemplary" in his role and considered him to be a "good friend." He added that he was "confident" that the Senate would confirm Kerry quickly.
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Among those murdered was Keith Palmer, an unarmed constable who "did his job and never wanted any fuss."
Top temperature 14 Celsius (57 F).
The Prime Minister will say Moscow's brazen flouting of international law represents a threat to the basis of democracy across Europe.