Regime air strikes killed nine civilians in the town of Jisreen early Tuesday, the Britain-based Syrian Observatory for Human Rights monitor said.

With the latest deaths, 95 civilians have been killed in regime bombardment of the battered enclave since early Monday, it said.

Russian-backed regime troops have advanced steadily since launching an assault on the last major rebel stronghold near the capital on February 18.

As of early Tuesday they controlled around 40 percent of the enclave after seizing a further area overnight, the Observatory said.

It reported clashes Tuesday in the northeast, centre and southeast of the enclave.

In total more than 780 civilians -- including 170 children -- have been killed in Eastern Ghouta since February 18, according to the Observatory.

The enclave's 400,000 residents have been living under siege since 2013, facing severe food and medicine shortages even before the latest onslaught. Aid deliveries on Monday had to be cut short amid continued fighting.

Regime warplanes pounded other areas of Eastern Ghouta including the main town of Douma early Tuesday, as well as the towns of Sabqa and Hammuriyeh overnight, the Observatory said.

The strikes on Douma reduced homes to piles of rubble on the sides of roads, an AFP correspondent there said.

An AFP reporter in Hammuriyeh said air strikes overnight targeted the town, with only a few residents emerging from the safety of their cellars after day broke.

Late Monday, the Observatory reported 18 people suffered breathing difficulties following a strike by a military aircraft in Hammuriyeh, without being able to specify the cause of the illnesses.

More than 340,000 people have been killed in Syria since the start of the civil war in 2011 with the brutal repression of anti-government protests.