A Syrian opposition leader has called on world leaders to take "immediate action" to end government attacks on rebel-held suburbs of Damascus, amid reports that about 150 people have been killed in government air strikes over the last 10 days.

Khaled Khoja, leader of the Syrian National Coalition (SNC), made the appeal on Thursday at a press conference held from the bloc's base in Turkey.

The government has been bombarding the eastern suburbs of Damascus known as Ghouta for days as part of a military campaign against rebel-held districts east and south of the capital Damascus.

Douma, east of Damascus, has taken the brunt of the air strikes.

"The Assad regime's killing of children and the elderly with rockets, barrel bombs, and toxic gases is as a crime as horrible as ISIL's slaughtering and burning of people alive," Khoja said, referring to the Islamic State of the Iraq and Levant (ISIL) group.

He said the "barbaric assault" on Douma by President Bashar al-Assad's forces constitutes a war crime and urged the UN to force the Syrian leader to stop indiscriminate attacks on rebel-held territory.

Government batteries have rained shells and rockets on Douma and other opposition-held districts outside the city in attacks that activists say have been some of the worst they have seen, including dozens of barrel-sized bombs that are dumped from helicopters.

The Britain-based Syrian Observatory for Human Rights said it has documented at least 146 people including 29 children who were killed in government air strikes in Ghouta since the beginning of the month.

Meanwhile, rapid advances by government forces backed by Lebanese Hezbollah fighters south of Damascus slowed on Thursday because of a snowstorm hitting the region.

Ahmad al-Masalmeh, an opposition activist in the southern city of Deraa, said snow fell in some areas south of Damascus, making it difficult for vehicles to move.

He also said there was no government air force activity.

Syrian troops and Hezbollah fighters on Wednesday seized strategic hills and villages from rebels and a local al-Qaeda affiliate near the Israeli-occupied Golan Heights in a surprise counteroffensive that alarmed Israeli officials.

Source: Agencies