(CNN) The Syrian rebels' assault on regime-held parts of Aleppo keeps getting deadlier, with 84 people killed over the past three days, the Syrian military said Monday.

The latest violence also wounded 280 people in western Aleppo, Syria's General Command of the Armed Forces said.

The military said rebels fired more than 100 mortars and launched 50 rockets. It also accused rebels of using chlorine gas on civilian areas, saying there were 48 cases of breathing difficulties.

Human rights groups that have long decried the regime's indiscriminate attacks say nothing justifies rebel attacks on civilians.

"The goal of breaking the siege on eastern Aleppo does not give armed opposition groups a license to flout the rules of international humanitarian law by bombarding civilian neighborhoods in government-held areas without distinction," said Samah Hadid of Amnesty International.

"Armed opposition groups have displayed a shocking disregard for civilian lives. Video footage shows they have used imprecise explosive weapons including mortars and Katyusha rockets, whose use in the vicinity of densely populated civilian areas flagrantly violates international humanitarian law."

The Syrian Observatory for Human Rights said at least 17 children have been killed in government-controlled western Aleppo since the offensive began.

And the United Nations special envoy to Syria, Staffan de Mistura, said he was appalled by the number of rockets indiscriminately launched by armed opposition groups on civilian suburbs.

Bunker-buster bombs

The regime, too, has been accused of indiscriminately attacking civilians in Aleppo. Even those hospitalized or living underground aren't safe.

"Bunker-buster" bombs destroyed the M10 hospital in rebel-held eastern Aleppo this month, opposition activists said. Much of the hospital was underground due to the near-constant fear of airstrikes.

Adding to the catastrophe: Bombings have also destroyed water infrastructure, depriving many Syrians of clean water.

UN investigation on chemical weapons

On Monday, the UN Security Council extended a mandate allowing an investigative panel to continue searching for the culprits of several chemical attacks in Syria.

The panel will remain standing until November 18 while a more concrete deal is negotiated for a one-year extension.

The probe was conducted by a UN Joint Investigative Mechanism that analyzed 8,500 pages of documents, transcripts of more than 200 interviews, and mounds of photos and videos equaling about 1,400 files. It considered 330 pages of forensic material and 3,500 other multimedia files.

All the information was voluntarily submitted to the mechanism or gathered from witness reports, according to the results of the investigation.

The panel did not act as a judicial body. Its goal was only to identify the "individuals and other actors involved in the use of chemicals as weapons and the roles that they played."

Its results: "The Syrian Arab Armed Forces had been involved in the use of toxic chemicals as weapons in three cases."

The mechanism linked the chemical attacks in Talmenes, in 2014; Qmenas, in 2015; and Sarmin, also in 2015, to the Syrian government.

But it also pointed to a second perpetrator: ISIS.

The United Nations found that the terrorist group "had conducted an attack on Marea using several artillery shells filed with sulfur mustard, a chemical weapon."