Lebanese Shiite fighters arrest a member of the Sunni-dominated Free Syrian Army in the town of Hatita, in the countryside of Damascus, Syria.

At least 80 rebels and regime fighters have been killed in six days of clashes northeast of Damascus, the Syrian Observatory for Human Rights monitor said Sunday.

The deaths came in clashes that began on Tuesday when two prominent Islamist rebel groups launched simultaneous attacks on three areas outside the capital.

The Jaish al-Islam and Faylaq al-Sham attacks have focused on the areas of Harasta, Dahiyat al-Assad and the region around Adra prison, the monitor said.

“Clashes since September 8 have killed 46 rebels and 41 regime forces, including from pro-regime militias,” said Observatory chief Rami Abdel Rahman.

On Wednesday, Jaish al-Islam also began an assault on Adra prison, Syria’s largest and where many opposition figures have been held.

It briefly captured two buildings in the women’s section of the prison, but was subsequently forced back.

In Damascus city meanwhile, the Observatory said one person had been killed and 10 wounded in rebel fire on the Bab Tuma district.

Rebel forces including Jaish al-Islam regularly fire rockets into Damascus from rear bases on the city’s outskirts.

The regime frequently carries out deadly air strikes on the rebel-held areas outside Damascus, often killing dozens of civilians at a time.

More than 240,000 people have been killed in Syria since the conflict began in March 2011 with anti-government demonstrations.