The US Secretary of State John Kerry said the continued violence in Syria showed that it was time for President Bashar al-Assad to go.
Speaking alongside Kerry, Foreign Secretary William Hague called for support for Syria's opposition to be increased significantly to bring the conflict to an end.
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It comes after security was stepped up around the country following the Manchester Arena attack which killed 22 people.
A murderer nicknamed 'Houdini' after escaping the death penalty seven times has been given the lethal injection.
The distress signal sparked a police hunt in the area to find the person or group who made it.