The Syrian Observatory for Human Rights said the fighters were killed while trying to arrest a former official in Assad's government, accused of issuing execution orders and arbitrary rulings against thousands of prisoners.
The fighters were from Hayat Tahrir al-Sham, or HTS, which led the stunning offensive that toppled Assad earlier this month.
Some reports say as many as 14 were killed.
Syria's transition has been surprisingly smooth but it's only been a few weeks since Assad fled the country and his administration and forces melted away.
The insurgents who ousted Assad are rooted in fundamentalist Islamist ideology, and though they have vowed to create a pluralist system, it isn't clear how or whether they plan to share power.
Many Syrians celebrated the end of Bashar Assad's rule after 14 years of civil war. (EPA PHOTO)
Activists and monitors say since Assad's fall, dozens of Syrians have been killed in acts of revenge, the vast majority of them from the minority Alawite community, an offshoot of Shiite Islam that Assad belongs to.
In the capital, Damascus, Alawite protesters scuffled with Sunni counter-protesters and gunshots were heard.
Alawite protests also took place along the coast of Syria, in the city of Homs and the Hama countryside. Some called for the release of soldiers from the former Syrian army now imprisoned by the HTS.
At least one protester was killed and five were wounded in Homs by HTS forces suppressing the demonstration, said the Syrian Observatory. In response to the protests, HTS imposed a curfew from 6 pm until 8am.
The Alawite protests were apparently sparked by an online video showing the burning of an Alawite shrine. The interim authorities insisted the video was old and not a recent incident.
Sectarian violence has erupted in bursts since Assad's overthrow but nothing close to the level feared after nearly 14 years of civil war that killed an estimated half-million people. The war fractured Syria, creating millions of refugees and displacing tens of thousands throughout the country.
This week, some Syrians who were forcibly displaced, started trickling home, trying to rebuild their lives. Shocked by the devastation, many found that little remains of their houses.
In the northwestern Idlib region, residents were repairing shops and sealing damaged windows, trying to bring back a sense of normality.
The city of Idlib and much of the surrounding province has for years been under control of the HTS, led by Ahmad al-Sharaa, formerly known as Abu Mohammed al-Golani, once aligned with al-Qaida, but has been the scene of relentless attacks by the government forces.
Syria's new authorities have burned a million Captagon pills and hundreds of kilograms of cannabis. (EPA PHOTO)
In Damascus, Syria's new authorities raided warehouses on Wednesday, confiscating drugs such as Captagon and cannabis, used by Assad's forces. A million Captagon pills and hundreds of kilograms of cannabis were set ablaze, the interim authorities said.
