Former UN Secretary-General Kofi Annan is stepping down as the UN-Arab League mediator in the 17-month-old Syria conflict at the end of the month.

In a statement, UN Secretary General Ban Ki-moon said: “Kofi Annan deserves our profound admiration for the selfless way in which he has put his formidable skills and prestige to this most difficult and potentially thankless of assignments.

“He has worked within the mandate provided to him by the General Assembly and with the cooperation of various Member States.

Mr Ban said "we have worked closely together these past months, and I am indebted to him and his team for all they have tried to achieve.

He said “I will continue to draw on his wisdom and counsel, and on the work of the Office of the Joint Special Envoy.”

Earlier, Russia said video footage of a public shooting by rebels of men apparently loyal to President Bashar al-Assad suggests there are human rights violations on both sides in the conflict.

The video, which was posted online, showed four men being put up against a wall and shot in the city of Aleppo.

Deputy Foreign Minister Gennady Gatilov said on Twitter: “The massacre of supporters of the government by the opposition confirms human rights violations are taking place on both sides."

He said: “It would be useful if Western and Arab politicians looked at the situation in Syria from this angle too. Everybody must stop violence”.

Opposition activists said Syrian troops killed at least 35 people, mostly unarmed civilians, when they shelled and overran a suburb of the capital Damascus yesterday.

Syrian state television said "dozens of terrorists and mercenaries surrendered or were killed" when the army raided Jdeidet Artouz and its surrounding farmlands.

Another resident of the suburb said the total number of dead was at least 50.

Reports from Syria cannot be independently verified due to state restrictions on foreign media.