A Syrian opposition leader called on world leaders to take “immediate action” to end government attacks on rebel-held suburbs of Damascus, amid reports that some 150 people have been killed in government airstrikes in the past 10 days.

Syrian government troops have captured a fiercely contested suburb of the capital Damascus.

UN secretary-general Ban Ki-moon has deplored the use of chemical weapons in Syria and demanded that those responsible are held accountable.

Chemical weapons were probably used in four locations in Syria this year, in addition to the confirmed attack near Damascus in August which forced the government to abandon its secret chemicals stockpile, UN inspectors have said.

Syrian President Bashar Assad said a United Nations report finding “clear and convincing evidence” that sarin nerve gas was used in Syria painted an “unrealistic” account, and he denied his government orchestrated the attack.

The US, Britain and France have said they believe that the Assad regime was responsible for a chemical weapons attack in Damascus last month.

UN inspectors say there is “clear and convincing evidence” that chemical weapons were used on a relatively large scale in the attack last month in Syria that killed hundreds of people.

We all like to believe that we live in societies that try to behave decently, ones that try to observe basic standards of humanity and justice.

Syrian rebels made a new push into Damascus today, clashing heavily with troops in the rebellious suburbs of the capital and firing mortars at a presidential palace and a Palestinian refugee camp, activists said.

Syrian warplanes are pounding opposition strongholds around Damascus and in the north, activists said today, as President Bashar Assad’s regime intensifies air strikes against rebels seeking to topple him.

Syrian soldiers killed 19 people in fighting to retake Damascus suburbs from rebels, activists said, a day after the Arab League suspended its monitoring mission in Syria because of mounting violence.