A monitor said Tuesday (March 20th) 36 pro-government fighters were killed in a district of Syria's capital as "Islamic State of Iraq and Syria" (ISIS) elements took control of it in a surprise nighttime attack, AFP reported.

There was no immediate comment from the government on the report by the Syrian Observatory for Human Rights, which could not provide casualty figures for the ISIS elements.

"ISIS took full control of Qadam, and 36 government troops and loyalist fighters have been killed," the Observatory said.

Dozens more were wounded or captured, or are still missing in action, Observatory chief Rami Abdel Rahman said.

ISIS launched the attack from positions it holds in the adjacent Hajar al-Aswad district, he said.

"Regime forces are bringing reinforcements to the area around Qadam to try to retake it," he added.

Qadam lies in a southern part of Damascus and has for several years hosted a range of Islamist opposition factions and extremists, including ISIS and rival alliance Tahrir al-Sham.

The Syrian government has used both military pressure and negotiated settlements to try to clear the area.

Last week, hundreds of Tahrir al-Sham fighters evacuated the district under a deal with Damascus that granted them and their family members safe passage, with most heading north-west to Idlib province.

ISIS elements have even agreed to evacuate the district in the past.

The group put out a statement late Monday saying it had captured most of Qadam, including areas "surrendered" to regime forces by Tahrir al-Sham.

The district is smaller than and not connected to Eastern Ghouta, an area east of the capital which is home to hundreds of thousands of civilians and opposition fighters.

Syrian troops are waging a separate offensive there.