David Cameron said Britain has spent over £300 million and "saved tens of thousands of lives" in Syria.
The Prime Minister said: "I've been to Jordan and seen one of the refugee camps myself and I know how much money and how much effort is going in.
"Britain, as ever, is a world leader at helping those who need help, and the people of Syria are right up at the front of that right now."
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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.