Syrian government air strikes killed at least 23 people including four children Thursday in the worst ceasefire violation yet in a key rebel bastion east of the capital, a monitoring group said.

Dozens more were wounded in the raids on Deir Al-Assafir, a town in the opposition stronghold of Eastern Ghouta, according to the Syrian Observatory for Human Rights.

The strikes, which killed 13 civilians, two police officers and eight unidentified victims, represented "the largest ceasefire violation in Eastern Ghouta," Observatory head Rami Abdel Rahman said.

A fragile ceasefire brokered by the United States and Russia has been in place across parts of Syria, including Eastern Ghouta, since February 27.

The truce does not include areas where the Islamic State group and Al-Qaeda's local affiliate Al-Nusra Front are present.

Eastern Ghouta is dominated by the powerful Jaish al-Islam (Army of Islam) rebel group, which has agreed to the truce, but there are also small pockets of Al-Nusra fighters.

Amateur footage filmed by activists and distributed online showed a man holding up the lifeless body of an infant wrapped in a white shroud.

"Are these fighters? They are children!" the man says in the video.

The Istanbul-based National Coalition opposition group condemned "the massacre carried out by regime forces against residents of Deir Al-Assafir".

In a statement, it called on the international community to take "serious, urgent measures" to address the latest violation.

While there have been occasional incidences of violence, some of them deadly, the ceasefire has largely been hailed as a success by the United Nations.

More than 270,000 people have been killed since Syria's conflict erupted in March 2011.