The Duchess of Cambridge, who is eight months pregnant, was all smiles at the Trooping the Colour parade - her last public engagement before she gives birth in mid-July.
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Many police are said to have been seriously wounded in the attack which may have been targeted at them, officials said.
Syrian government forces are continuing a push for total control of Aleppo, while a reported 20,000 civilians left the city on Saturday.
Rex Tillerson is president of Exxon Mobil and has close business links to Russian President Vladimir Putin.