Smoke rises from the northern Syrian town of al-Bab, Feb. 2, 2017. Reuters

Turkey-backed Syrian rebels opened fire Sunday on Syrian army forces and allies advancing toward opposition-held positions as the opposing factions waged parallel campaigns against the Islamic State group, also known as ISIS, in the country's north.

Various insurgent groups have attempted to overthrow Syrian President Bashar Assad since 2011 when massive protests transformed into an armed uprising against the state. In recent weeks, Russia, which backed Assad, and Turkey managed to broker an unsteady truce between their sponsored forces to concentrate on defeating ISIS, the common enemy of the government and rebel forces, in the northwestern city of al-Bab. Tensions between the Syrian army, its allied militias and rebels previously manifested themselves in clashes earlier this month and the latest confrontation has highlighted the failure of peace talks being held in Geneva and Astana, Kazakhstan, to achieve reconciliation between the government and opposition in Syria.

Sunday's violence reportedly occurred when the Syrian army and allied militias approached rebel positions in al-Bab, recently captured from ISIS by militants fighting under the banner of the Free Syrian Army. The Syrian army and allies were based in the town of Tadef, less than 2 miles southeast of al-Bab, where rebels targeted pro-government forces "with artillery and machine guns" before Russia mediated, an unnamed Syrian military official told Reuters, without specifying the number of casualties inflicted. An opposition statement claimed "22 regime members" killed, referring to soldiers loyal to Assad.

The Syrian military source said the attack proved the rebels' "main aim is not fighting Daesh, but realizing other goals including an attempt to obstruct the Syrian army's operation confronting Daesh," using the Arabic-language acronym for ISIS. Assad, his supporters and a number of the oppositions' critics including President Donald Trump, have accused nations that back rebel forces of bolstering ISIS' ranks. The Pentagon, which has supported the opposition to varying degrees throughout the conflict, recently halted assistance to rebels after years of leaks to jihadist forces such as Tahrir al-Sham, formerly known as Jabhat Fatah al-Sham and al-Qaeda's Nusra Front before that, and ISIS.

The Free Syrian Army has denied any connection to ISIS but has openly coordinated with some Islamist groups, especially as foreign support from the West declined and backing from Gulf countries such as Saudi Arabia and Qatar favored more conservative hardline organizations. Infighting among rebel groups, along with direct Russian intervention in 2015, has given the Syrian army the upper hand in the conflict. The government is in control of virtually all of Syria's major cities and has attempted to negotiate a solution to the six-year conflict that has killed hundreds of thousands and displaced millions more.

The news came as the Syrian army fought Monday with ISIS near the besieged town of Deir al-Zour in eastern Syria and bombed militant positions in the northwestern, rebel-held city of Idlib, Syrian state media reported.