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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Rain will spread eastwards across southern areas tonight. Clear and frosty in the north.
Political leaders will go head-to-head at ITV's studios in MediaCityUK in Salford, ITV has confirmed, as the set for the event is unveiled.
More than 100 people claiming to represent working people across Britain have signed a letter backing Labour in the general election.