At least 80 people, including women and children, have been killed in Damascus, according to Syrian activists.
Many were reportedly executed by government forces at a makeshift hospital in the town of Jdeydet al-Fadel, the Syrian Observatory for Human Rights (SOHR), the UK-based network, said on Sunday.
Al Jazeera has been unable to independently verify the report, but has been sent video images of the bodies.
The report came as the leader of the main Syrian opposition group offered his resignation from the post yet again.
Moaz al-Khatib, president of the Syrian National Coalition (SNC), submitted his resignation, a statement on the organisation's Facebook page announced on Sunday.
The SNC said it would take up the matter at its next meeting, without providing a date.
Khatib, a respected Muslim preacher seen as a uniting figure and moderate, tried to quit his post in March, citing frustration over what he called a lack of international support and constraints imposed on the body itself.
The coalition rejected his resignation then, and he agreed to stay on until his six-month terms ends in May.
Claiming that scores of bodies were found in Jdeydet al-Fadel, in the suburbs of Damascus, the SOHR said it was able to document the names of 80 victims and that the death toll might be much higher.
Opposition fighters pulled out of the town on Saturday because they ran out of ammunition, the SOHR said.
By Sunday morning, government forces had taken full control of the area.
The killings reportedly took place during four days of fighting between government forces and anti-regime fighters.
In-depth coverage of escalating violence across Syria
Many were killed during shelling and in summary executions, according to the SOHR.
Violence has also raged in Sunni Muslim areas of the nearby Christian-majority town of Jdeydet Artuz, and in the rebel stronghold of Daraya, the scene of fierce fighting for several months.
The SNC accused the army of staging a "fierce attack" in areas south and west of Damascus.
Jdeydet al-Fadel and other flashpoints are "subjected to a siege and they are deprived of all basic needs for human life", the SNC said on Saturday.
Since last year, government forces have tried to root out rebels positioned southwest and east of Damascus in an effort to secure the Syrian capital.
In another development, the SNC called on Hezbollah to withdraw its fighters from the country, as activists said regime troops supported by the mainly Shia Muslim Lebanese group battled rebels on Sunday for control of a string of villages near the Lebanon-Syria border.
The SNC cautioned that Hezbollah's involvement in Syria's civil war could lead to greater risks in the area, and urged the Lebanese government to "adopt the necessary measures to stop the aggression of Hezbollah" and to control the border to "protect civilians in the area".
The SNC statement coincided with a surge in fighting around the contested town of Qusair in Syria's Homs province, near the frontier with Lebanon.
Over the past two weeks, the Syrian military, supported by a Hezbollah-backed group, has pushed to regain control of the border area.
The region is strategic because it links Damascus with the Mediterranean coastal enclave that is the heartland of President Bashar Assad's Alawite sect, an offshoot of Shia Islam.
The pro-regime fighters are members of the Popular Committees, which were set up last year in Syria with Hezbollah's backing to protect Syrian villages inhabited by Lebanese Shias, although rebels accuse the fighters of attacking opposition villages in the area and fighting alongside government forces.
The fighting along the border region has flared in recent weeks, and on Saturday government forces captured the villages of Radwaniyeh and Tel al-Nabi Mando.
Regime forces shelled the villages of Abu Houri, Saqarigh, Nahriyeh and Ein al-Tanour in the Qusair region on Sunday, according to the SOHR.
It said at least four rebels were killed in the fighting.