DUBAI, Sept 13 (Reuters) - Qatar, a leading supporter of Syrian rebels fighting to topple President Bashar al-Assad, welcomed a ceasefire in Syria, saying compliance would hasten a political solution and facilitate humanitarian help.

The Syrian Observatory for Human Rights said on Tuesday the truce, brokered by Russia and the United States and which began at sundown on Monday, was holding across Syria.

QNA said Qatar's foreign ministry, in a statement on Monday evening, "expressed hope that the implementation of the agreement would contribute to reaching a comprehensive and permanent political settlement to end the Syrian crisis and lift the humanitarian suffering from the Syrian people ... ".

Qatar also urged world powers to ensure Syrian government forces implement the agreement.

Other Gulf Arab states, including regional power and main rebel supporter Saudi Arabia, have yet to comment on the ceasefire agreement.

Russia and Iran are both providing crucial military support to Assad against rebels and jihadi fighters. Iran has sent what it said were military "advisers" to help Assad and allowed Russian fighter-bombers to use an Iranian base to launch operations in Syria in August.

(Reporting by Dubai newsroom; Editing by Sami Aboudi) ((dubai.newsroom@thomsonreuters.com; +971 4 3918301))