Rebels have expelled jihadists from four areas in the Damascus region, after having lost territory to the Islamic State in the north and east of Syria, a monitoring group said Monday.
IS, meanwhile, has started selling Syrian oil to Iraqi businessmen, according to the Syrian Observatory for Human Rights.
On the diplomatic front, Foreign Minister Walid Muallem told his Russian counterpart he was certain of the regime's "victory" in Syria's civil war, thanks mainly to Moscow's support.
Despite being besieged by government troops, rebels have expelled the IS from four areas in the Damascus region in a drive launched three weeks ago, the Observatory and rebel sources said.
The monitoring group said clashes were ongoing in other areas south of Damascus.
IS was initially welcomed as a potential ally in the armed revolt, but the opposition, including Islamists, has turned against the radical group.
The Observatory said IS fighters have been expelled from Mesraba and Maydaa, two towns in the Eastern Ghouta of Damascus.
They had also been forced out of Yalda and Beit Sahem.
The expelled jihadists fled to Al-Hajar Al-Aswad, Tadamon and Qadam in southern Damascus, "where they have a strong presence", said Observatory director Rami Abdel Rahman.
On Monday, fighting between rebels and jihadists broke out in the battered neighbourhoods of Al-Hajar Al-Aswad and Qadam, he told AFP.
A rebel spokesman in Damascus province confirmed the reports.
"For us, there was no choice but to fight IS," said Captain Abdel Rahman al-Shami, a spokesman for the rebel Army of Islam, spearheading the offensive in the Damascus area.
"It was in self-defence. We are in a suffocating (regime) siege. We are fighting the regime, while IS is shooting us in the back," Shami said.
"Now, they no longer have any official bases in Eastern Ghouta. But we are chasing the remnants of IS," he told AFP via the Internet.
- Thanks to Moscow -
Ever since Syria's revolt morphed into an armed rebellion, the Eastern Ghouta area has remained outside the regime's control.
On Monday, Muallem told Russian Foreign Minister Sergei Lavrov in a letter that Damascus believes its "victory" is assured, thanks to the support of Moscow and other allies.
"Syria's unwavering confidence in its victory is thanks to its people's resistance, and to the support of its friends, especially Russia," said Muallem.
"Syria is more determined than ever to crush the terrorists and to defend its sovereignty... and the security of the region," he added, referring to the anti-regime revolt which has cost more than 170,000 lives since March 2011.
The Observatory, meanwhile, reported that jihadists are selling oil and liquid gas products extracted from fields under its control in Syria to Iraqi businessmen across the border.
"Each barrel of oil is sold to Iraqi businessmen for $20 to $40," compared to more than $100 a barrel on global markets, Abdel Rahman said.
His group said IS, which last month declared a "caliphate" straddling Syria and Iraq,  was also selling oil to Syrians living in areas under its control for $12 to $18, to win the support of locals.