Rebels in southern Syria announced a major offensive on Wednesday to capture remaining positions held by the Syrian military in Quneitra province, near to the Israeli-occupied Golan Heights.
Rebel spokesman Issam al-Rayes wrote on Twitter that insurgent groups fighting in the operation had signed a pact, which did not involve Al-Qaeda's Syria wing Al-Nusra Front.
Rayes posted a picture of what he described as an operations room in Quneitra showing a group of men in military fatigues close to a table that appeared to show a map and equipment.
Quneitra sits in a sensitive piece of territory around 70 kilometres (40 miles) southwest of the capital Damascus and has been the scene of frequent fighting between various insurgent groups and the Syrian military backed by allied militia.
The groups in the offensive are fighting under the banner of the rebel Free Syrian Army. Al-Nusra has fought in southern Syria but is not thought to be the main insurgent force there, unlike in other parts of the country.
A Reuters photographer watching from the Israeli-occupied Golan said there had been heavy shelling since early Wednesday in the Quneitra area, with scores of bombardments.
At one point the photographer could see smoke rising from around 13 bombardments and the sound of small arms being fired could also be heard in the distance.
The shelling appeared to be focused between Quneitra's water reservoir and the town itself, with some buildings on the outskirts appearing to have suffered damage, the photographer said.
Meanwhile, the Syrian Observatory for Human Rights reported Wednesday the death of four children and three other civilians in renewed rebel rocket fire on government-held areas of Syria's second city Aleppo.
Tuesday's fire came despite U.N. condemnation of a massive barrage the previous day that killed at least 36 civilians, 14 of them children.
The monitoring group said there were also "many" wounded but did not immediately give a figure.
Different groups, including the hardline ISIS and Al-Nusra Front, have been putting the Syrian military under heavy pressure in various parts of the country in the past two months.
The government says it can defend important stretches of territory in Syria's populous west and the deputy foreign minister told Reuters last week that Damascus was safer than towards the start of the conflict, which grew out of protests against Assad in 2011.