Jihadists have made advances on rebel fighters in the battle for Raqa, in northern Syria, but are on the back foot in parts of Aleppo and Idlib, activists said Friday.
"In Idlib and Aleppo provinces, the (rebel) Free Syrian Army is advancing, but in Raqa the Islamic State of Iraq and the Levant (ISIL) is winning because its supply routes (to Iraq) are open there," said Alaaeddine, an Aleppo-based activist.
"In Idlib there are practically no ISIL bases left, as is the case in Aleppo city and the west of the province" on the border with Turkey, he told AFP via the Internet.
But in Raqa, which came under ISIL's control soon after President Bashar al-Assad's regime lost control of the provincial capital, "ISIL seized the Mashlab district and an Al-Nusra Front base" on Thursday night, said the Syrian Observatory for Human Rights.
ISIL has its roots in Al-Qaeda in Iraq, and first appeared in the Syrian conflict in late spring last year.
While rebels initially welcomed the jihadists in Syria, ISIL's horrific abuses and quest for hegemony has turned "90 percent of people" in opposition areas against the group, said Alaaeddine.
Fighting pitting ISIL against other rebel groups -- including Al-Nusra Front, which is also linked to Al-Qaeda but is seen as more moderate -- broke out last Friday.
Hundreds of fighters on both sides have since been killed, according to the Britain-based Observatory, which relies on a broad network of activists and doctors on the ground for its reports.
Civilians have also suffered as a result of the latest fighting.
"In Aleppo city, people are trapped in their houses, unable to fetch medicine or food, for fear they will get shot by snipers if they go outside," said Alaaeddine, adding that "in Raqa, the situation is even worse".
Even as the fighting raged in Aleppo province, several opposition neighbourhoods of the city came under renewed aerial attack by regime loyalists, said the Observatory.
Meanwhile, rebels fought troops loyal to Assad, including Lebanese Shiite Hezbollah fighters, in Taqarin and Tal al-Sheikh Yusef villages of Aleppo province, it added.
Syria's war broke out after Assad unleashed a brutal crackdown against pro-democracy uprising that erupted in March 2011. The war has killed more than 130,000 people and forced millions more to flee their homes.