(CNN) -- After another violent week in Syria's civil war, U.N. Secretary-General Ban Ki-moon expressed regrets for the "inability of the international community to unite in stopping this conflict."
Ban was referring to a mortar attack Friday on an electoral rally that reportedly killed at least 21 people.
A child was among the dead in the shelling, which struck a pro-government election campaign tent in the regime-controlled part of Daraa.
The opposition Syrian Observatory for Human Rights said that Islamist rebels were responsible for the attack and that at least 11 of the victims were civilians.
The United Nations has asked the Syrian government to reconsider next month's election as the carnage mounts. Syrian President Bashar al-Assad, who is blamed for the using his military to clamp down on opposition forces, is once again a candidate.
The observatory estimates that more than 160,000 lives have been lost in the conflict.
"The Syrian people desperately need an end to violence and a clean break from the past to move toward a new Syria, one in which their legitimate aspirations are met and all communities are protected," said a representative for Ban.
But the latest reports from the ground do not show signs of a slowdown in violence.
Syrian troops lobbed shells laced with chlorine at a western Syrian town on Thursday, causing dozens to suffer from the inhalation of toxic gas, the opposition-run Hama Media Center said.
A dramatic video posted on social media by activists purportedly shows yellow gas hanging in a narrow street at dusk in Kafr Zeita.
A young man in a gas mask runs into the cloud of smoke and walks out moments later with a woman trying to cover her mouth as she struggles to breathe.
Another video shows children choking and women using oxygen masks at a makeshift clinic. A man in scrubs turns to the camera while treating the victims and says, "This is a chlorine gas attack."
Kafr Zeita.is the same opposition-held location where the United States and its allies believe al-Assad's government used the toxic gas in an April 11 attack.
CNN cannot independently confirm the authenticity of the videos.
Also Thursday, the Syrian government wrested control of the Central Prison in Aleppo, breaking a 13-month rebel siege on the sprawling complex where dozens of political prisoners are believed to be held, according to opposition groups and Syrian state media.
Since a 2012 prison revolt, detainees suffered from lack of food, inadequate medical care and the spread of infectious diseases such as tuberculosis and dysentery, the Violations Documentation Center, a local monitoring group, found in a report published in April.
More than 650 prisoners died either due to deteriorating conditions or at the hands of jailers, whom the center alleges carried out torture and summary executions.
CNN's Saad Abedine contributed to this report.