Beirut (AFP) - Syrian and Russian aircraft on Tuesday pummelled a jihadist holdout in southern Syria bordering Jordan and the Israeli-occupied Golan Heights after deadly strikes overnight, a Britain-based monitor said.

Late Monday, Russian air strikes killed four civilians in the pocket held by the Islamic State group in the southern province of Daraa, the Syrian Observatory for Human Rights monitor 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 Moscow's warplanes, Syrian regime forces are battling to clear out a final IS-held patch of the country's south after ousting other rebels.

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 IS branch known as Jaish Khaled bin Walid, he said.

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

IS and other jihadists are not included in the ceasefire deals that rebels have agreed with regime ally Russia across the south, which provide for an end to bombardment if rebels 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 rebel control in the country's north.

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

Daraa is considered to be the cradle of the 2011 uprising that spiralled into civil war.

The regime has retaken large parts of Syria from rebels and jihadists since Russia intervened military to back President Bashar al-Assad's fighters in 2015.

More than 350,000 people have been killed and millions displaced since the conflict started in 2011 with a brutal crackdown on anti-government protests.