Beirut: Warplanes have launched heavy attacks on the two last major rebel-held areas in Syria, killing at least 29 people in the Ghouta suburb near the capital and choking people with gas in Idlib in the north-west, rescue workers and a war monitor says.

President Bashar al-Assad's government has vowed to retake all of Syria from rebels who have lost large swathes of the territory they have held in a war now entering its eighth year.

A years-long siege on the last major rebel-held area near the capital Damascus, the suburb of eastern Ghouta, has tightened in recent months. In the north-west, the government and its militia allies have been trying to advance in mostly rural Idlib, the last province still largely under rebel control.

An injured boy receiving treatment at a hospital in Hazeh in eastern Ghouta, the only remaining rebel stronghold near the capital, Damascus, on Monday. Credit:AP

The Britain-based Syrian Observatory for Human Rights monitoring group said warplanes attacking eastern Ghouta near Damascus had struck the towns of Zamalka, Arbaeen, Hazza and Beitu Soua, killing at least 29 people. State media said rebel fighters shelling the government-held capital killed a woman.