At least 26 civilians were killed in an army crackdown on defectors in Syria's southern province of Daraa  late Wednesday evening, activists said Thursday.
In total, at least 57 people were killed in the assault, which came a day after army members defected to the opposition in the area.
The Syrian Observatory for Human Rights said Thursday that the figure includes six children and seven women.
"The defectors took refuge in the area of Al-Sanamein and Ghabagheb, which up until then had remained somewhat more calm than other areas of Daraa," Observatory spokesperson Rami Abdel Rahman told AFP.
In the ambush, 16 rebels were also killed, the Observatory reported, as were 12 regular soldiers, and three army defectors, in a combination of shelling, fighting and summary executions.
Several houses were destroyed in the bombardment, the Observatory reported, and residents were detained.
The crucial southern border area – where the uprising initially began over two years ago – has become an increasingly key battleground for rebels over recent weeks.
Control of the area would help secure the road to Damascus, and control of the capital, where President Bashar Assad’s most loyal troops and supporters reside, reaching which is arguably the rebels’ end goal.
Last week, rebels captured a military base outside the city of Deraa . This followed the capture of Dael, one of the province’s larger towns, and a Syrian army air base on the Damascus-Amman highway in late March.
Across the country Thursday, regime troops continued to shell the suburbs of Damascus, in an effort to hold back rebels who control several suburbs of the capital.
The Local Coordination Committees, another activist group, also reported regime shelling in Deir Ezzor, Idlib, Aleppo, Hama and Homs, where at least three children had been killed by midday.
In London, the Syrian crisis is high on the agenda at a G-8 meeting. 
U.S. Secretary of State John Kerry  and British Foreign Secretary William Hague Wednesday held a meeting with Syrian rebel representatives, who renewed calls for increased assistance.
The U.S. and the EU currently provide non-lethal aid and some training to the rebels but the U.K. and the France, in particular, have been pushing for the lifting of an arms embargo on the opposition.
However, Kerry “did not promise anything” at Wednesday’s lunch, AFP reported.
"We are always considering a variety of options, we are going to continue to aid the opposition, working with them in terms of what they need, in terms of what we're willing to provide," a top State Department official said.
There are concerns over a proliferation of weapons in Syria, particularly among certain opposition groups, such as the Nusra Front, which the U.S. last year blacklisted as a terrorist organization.
Al-Qaeda in Iraq this week said that it had merged with the Nusra Front, a statement the Syrian group appeared to distance itself from Wednesday. However, Abu Mohammad al-Golani, the leader of the Nusra Front, also pledged allegiance to Al-Qaeda chief Ayman al-Zawahri.