A Syria donors' conference in London next week will urge participating countries to double the amount of money they are giving to tackle the humanitarian crisis, Downing Street said Wednesday.

On a call between the leaders of Britain, Germany and Norway, they agreed "that all countries in attendance should look to at least double their 2015 financial contribution to the crisis," Prime Minister David Cameron's office said in a statement.

Britain, Germany and Norway are co-hosting the event with the United Nations and Kuwait.