Force comprising 43 groups, under the Islam Army banner, will be the biggest to operate on the outskirts of Damascus.
At least 12 people, most of them students, have been killed in an air attack that hit a secondary school in the rebel-held Syrian city of Raqqa, opposition activists say.
Graphic amateur video said to be filmed on Sunday in Raqqa, in northeastern Syria, showed several bloodied bodies strewn across a dirt yard, some torn in half.
Opposition activists based in Raqqa, a city of around 250,000 people, said there were more than 30 people wounded besides the fatalities.
Al Jazeera cannot independently verify the reports due to reporting restrictions inside Syria.
Raqqa has been under the control of fighters battling to overthrow President Bashar al-Assad since March but the city remains subject to regular aerial bombardment by government forces.
Further south, government forces shelled near the Ramtha border crossing in Deraa near Jordan on Sunday, a day after rebels- including fighters affiliated to al-Qaeda- seized control of the post.
The Ramtha post served in the past as the customs office on the border with Jordan.
Al Jazeera’s Nasser Shadid, reporting from Deraa, said that while the border crossing was now under the control of the rebels, the surrounding area was still under under the control of regime troops.
“So clashes are continuing between the two sides,” he said.
Protest over shelling
The Jordanian state news agency reported on Sunday that a shell from the fighting in Syria fell landed in Jordan on Thursday night, prompting the Foreign Ministry to send a written protest to the Syrian embassy in Amman.
In August last year, four rockets fired from Syria crashed inside a Jordanian northern border area, wounding a young girl and sparking an earlier letter of protest.
Jordan is home to more than 500,000 Syrian refugees.
Meanwhile, in the suburbs of Damascus, opposition groups upheld a ceasefire between 11am and 12pm in towns under their control, Al Jazeera’s Mahmoud Zaibak reported on Sunday, following an agreement with UN monitors, who returned to Syria last week to investigate sites of alleged chemical-weapons attacks.
He said the lull in fighting was implemented in the town of Zamalka and the Damascus districts of Qaboun and Jobar.
Also on Sunday, at least four vehicles belonging to the UN inspectors were seen returning to the Four Seasons Hotel in Damascus after they left in the morning. Their destination was not known.
The experts would investigate seven sites of alleged chemical attacks in the country, four more than previously known.
The UN said the team expects to finalise its activities in the country by Monday.
The inspectors, who visited Syria last month, determined that the nerve agent sarin was used in an attack of the Damascus suburb of Ghouta on August 21, but it did not assess who was behind it.
The UN Security Council adopted on Friday a resolution that demands the elimination of Syria’s chemical-weapons arsenal.
Assad on Sunday told Italian television station RaiNews24 that his country would respect the resolution.
“We joined the international agreement against the acquisition and use of chemical weapons even before this resolution was passed,” he said when asked if Syria would comply with the UN resolution.
The measure is the first UN resolution on the Syria’s 30-months conflict, which the UN says has left more than 100,000 dead.