Beirut (AFP) - Shells fired by Syrian rebel groups killed at least 28 civilians in southwestern districts of the battleground city of Aleppo over the last 24 hours, a monitor said late Monday.

Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, said those killed "included six children and eight women" in two government-controlled neighbourhoods of divided Aleppo.

He said dozens of people were also wounded.

The city's southern edges have been ravaged by intense fighting in recent days as rebels seek to ease a government siege and cut off the regime's own access route into the rest of the northern province.

Syrian state news agency SANA said that since Sunday, 20 civilians were killed and dozens wounded in shelling, rocket fire and sniper attacks on government-held neighbourhoods.

It said nine people -- among them three children -- were killed on Monday and 11 people died in the attacks on Sunday.

Once Syria's economic powerhouse, Aleppo city has been roughly divided between government control in the west and rebel control in the east since mid-2012.

In recent weeks, government forces have encircled the east, raising fears of a humanitarian crisis for the estimated 250,000 people now under siege there.

Last week, Moscow announced the opening of "humanitarian corridors" from the east into government territory for civilians and surrendering rebels.

More than 280,000 people have been killed since Syria's conflict erupted in March 2011 with anti-government protests that morphed into a brutal civil war.