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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Joshua Clements stabbed two 19-year-olds while 4,000 people were gathered at the royal park.
The vessel was spotted off the Kent coast at around 5.40am on Thursday.
The US secretary of state called for a new approach to resume talks amid the civil war, including unity government and transfer of weapons.