A total of 21 UN observers left Syria Monday as the mission's extended mandate will expire on August 20, sources told Xinhua.

The reason behind the observers' departure remains officially unknown, however, sources told Xinhua that the head of the mission Gen. Babacar Gaye has taken the decision after consultations with the UN Security Council.

A day earlier, the mission's spokesperson told Xinhua that there are 150 UN observers currently operative in Deir-al-Zour, Homs and the capital Damascus.

The UN Supervision Mission in Syria was extended for a final 30 days mandate in July. Shrinking the 300-observer mission into half was part of the extended mandate.

The mission has done little to stem the simmering violence in the unrest-hit Syria.