Syria has dismissed calls by the United States and its allies for President Bashar al-Assad to step down as pitiful and illegitimate. The Syrian ambassador to the UN accused the US of launching a humanitarian and diplomatic war against his country. The United Nations is to send a humanitarian mission to Syria on Sunday. Earlier, the leaders of the United States, France, Germany and Britain had demanded that President Assad of Syria leave office following the violent suppression of street protests against his leadership. President Obama said, the time has come for President Bashar al-Assad to step down. In a joint statement, the leaders of Britain, France and Germany said, President Assad should leave power in the greater interests of Syria and the unity of the people. A Syrian Government spokesman accused the Western Governments of increasing tension in the country. He said, it is strange that instead of offering Damascus a helping hand to implement its programme of reforms, the West and President Obama are stoking more violence in Syria. Syria's ambassador to the United Nations, Bashar Jaffery has said, Syrian security forces have stopped all operations.