UN weapons experts are due to inspect a site where reportedly poison gas killed hundreds of people in Damascus suburbs, amid calls from the West for action.
Syria agreed on Sunday to allow the inspectors to visit the site. Foreign Secretary William Hague has said the evidence to prove the Assad regime carried out a chemical attack on Syrian civilians may have been destroyed in the days since.
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At least 13 people are said to have been killed by a car bomb outside a Turkish stadium in an attack which reportedly targeted the police.
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.