The cornerstone of their efforts - a 165-strong peace monitoring effort - was thrown into doubt on Tuesday when Gulf Arab states began withdrawing 55 of their monitors, saying they had failed to stem the violence. The deputy head of the 22-member Arab League said the remaining monitors would continue their work. However, Arab League chief Nabil Elaraby and Qatari Prime Minister Hamad bin Jassim al-Thani, who heads the organisation's committee on Syria, wrote to UN Secretary General Ban Ki-moon requesting a meeting to seek the Security Council's support for the League's faltering peace effort. One ambassador to the League said that bringing in the United Nations Security Council could lead to a tougher set of measures if Assad fails to comply. "The Syrian regime did not implement the Arab plan under existing Arab pressure, so there was no other way except to approach the Security Council," the ambassador at the League said before the announcement of the letter and on condition of anonymity. The League's new plan agreed at the weekend calls on Assad to step down, transfer power to his deputy and allow the formation of a unity government. If the plan is adopted, he said, the Security Council would determine measures to push Syria to comply, such as economic sanctions similar to those imposed by Arab states, an arms embargo or restrictions on movements by Syrian diplomats.