The six, including two men and their wives, were killed in the strikes on the Palestinian camp of Yarmuk late Tuesday, the Syrian Observatory for Human Rights monitoring group said.

Regime strikes and rocket fire Wednesday morning targeted the neighbouring districts of Hajar al-Aswad and Qadam, the Britain-based monitor said.

The latest civilian deaths bring to 18 the total of non-fighters killed in regime bombardment on the capital's southern neighbourhoods since Thursday last week.

Yarmuk, which is now IS's last urban redoubt in Syria or Iraq, was once Syria's biggest Palestinian refugee camp, home to around 160,000 people.

But the United Nations' agency for Palestinian refugees UNRWA says most of the 6,000 refugees still living in the camp last week have since fled.

At least 52 pro-regime fighters have been killed in fighting to expel IS from the capital's southern suburbs since April 19, the Observatory says.

Syrian officials do not usually disclose losses within army ranks.

The monitor has said at least 35 jihadist fighters were also killed during the same period.

There are an estimated 1,000 IS fighters left inside Yarmuk and the adjacent districts of Hajar al-Aswad and Qadam.

IS swept across large parts of Syria and neighbouring Iraq in 2014, declaring a cross-border "caliphate" in areas the jihadists seized.

At its height their pseudo-state covered an area the size of Italy, but IS has since lost most of the land it controlled in both countries.

More than 350,000 people have been killed since Syria's war started in 2011 with the brutal repression of anti-regime protests.