Syrian and Russian aircraft on Tuesday (July 24th) pummelled an extremist holdout in southern Syria bordering Jordan and the Golan Heights after deadly strikes overnight, AFP reported.

Late Monday, Russian airstrikes killed four civilians in the pocket held by the "Islamic State of Iraq and Syria" (ISIS) in the southern province of Daraa, the Syrian Observatory for Human Rights said.

The latest deaths bring to 41 the number of civilians killed in bombardment on that pocket in less than a week, it said, 11 of them children.

Backed by Russian warplanes, Syrian regime forces are battling to clear out a final ISIS-held patch of the country's south after ousting other opposition groups.

On Tuesday, regime helicopters and Russian war planes carried out heavy bombardment of that area, causing widespread damage, Observatory head Rami Abdel Rahman said.

"Fierce fighting raged in the north and west of the region" controlled by a local ISIS branch known as Jaish Khalid ibn al-Walid, he said.

Since Thursday, clashes between both sides have killed at least 43 regime fighters and 61 ISIS elements, the Observatory said.

ISIS and other extremists are not included in the ceasefire deals that opposition fighters have agreed with regime ally Russia across the south, which provide for an end to bombardment if opposition fighters hand over heavy weapons and territory.

Opposition fighters who oppose a regime takeover can board buses with their families to be ferried to areas still under opposition control in the country's north.

In the latest of these transfers, buses carrying 900 fighters and relatives from the town of Busra al-Sham in Daraa arrived in north-western Syria on Tuesday, the Observatory said.