Syrian regime forces advanced in Eastern Ghouta and pounded two towns with airstrikes on Monday (March 12th) as they moved closer to retaking the opposition enclave on the edge of Damascus, AFP reported.

Meanwhile, the UN warned Monday, more than 1,000 people, mostly women and children, are in urgent need of medical evacuations from the enclave.

Three weeks after launching a fierce offensive to capture the region, the regime and its allies have overrun more than half of the enclave, whittling down opposition-held territory to three isolated pockets.

The advance, one of the most ferocious of Syria's civil war, has battered the enclave with airstrikes, artillery and rocket fire, raising widespread international concern and prompting calls for a ceasefire.

Pro-regime forces advanced again on Monday, using Medeira, a town captured on Sunday, as a base to push against areas closest to the capital, including Harasta and Arbin, according to the Syrian Observatory for Human Rights.

Regime airstrikes on Monday were pounding the two towns, it said.

The other two areas still in opposition hands are Douma, in the north of the enclave, and the zone around Hammouriya and other towns to the south.

US Defence Secretary Jim Mattis said Sunday it would be "very unwise" for the regime to use gas as a weapon against the people of Eastern Ghouta and elsewhere.

His warning came amid reports that the regime has used chlorine gas in the enclave.