(CNN) Around 100 people were injured in toxic gas attack on the government-controlled city of Aleppo Saturday, according to Syrian state media and a British-based human rights group.

It's not clear who carried out the attack, but Syria's state news agency quoted a local commander as blaming "terrorist groups," while Russia accused militants of firing mortar rounds containing chlorine on the city's northwestern districts.

Nasr al-Hariri, the head of the Syrian opposition negotiation delegation to Geneva, told Saudi owned al-Hadath TV that the attack on Aleppo originated from an area controlled by pro-regime Iranian militias.

On Twitter he wrote that "the regime and its allies are using this as an excuse to launch a military operation in north Syria."

The reported attack on Aleppo is the first since Russia and Turkey agreed to establish a demilitarized zone in the rebel-controlled Idlib province, according to UK-based Syrian Observatory for Human Rights (SOHR).

Rights groups said an investigation into the attack was needed and called on Russia and Syria to allow a neutral third party to look into it.

"Serious reports of suspected chemical weapon attacks should not be left without investigation," Lama Fakih, Deputy Director for the Middle East and North Africa, Human Rights Watch said in a statement.

"All parties, including the Syrian government and Russia, should facilitate an investigation by the Organization for the Prohibition of Chemical Weapons into the suspected attack in Aleppo which reportedly affected dozens, including children. All parties should stop the use of prohibited weapons and attacks on civilians."

The Syrian army responded by shelling the source of the attack, SANA reported, without providing additional details. Russian warplanes hit Syrian rebel targets, state news agency RIA-Novosti reported.

Toxic air

SOHR said 94 civilians, including dozens of children, suffered from asphyxiation following the toxic gas attack in Aleppo. The group said explosions had been heard as shells fell on the western part of the city.

Citing hospital sources, SANA reported 107 people were injured in the gas attack, ranging from mild to medium in severity.

Russian state media also reported injuries from the attack, saying that 46 people, including eight children, suffered from exposure to gas. It added that Russian chemical weapons specialists had been deployed to provide relief to the city's affected residents.

Other gas attacks

The attack, if confirmed, would not be the first time gas has been used in the Syrian conflict.

The groups -- including the White Helmets, the Douma Coordination Committee and the Ghouta Media Center -- said toxic gas inside barrel bombs dropped by helicopters over Douma caused people to suffocate and choke.

Syrian state news agency SANA cited an "official source" who denied the allegations, and citing the same sources wrote that the Syrian Arab Army "does not need to use any chemical materials as claimed by terrorists' media affiliates."

First attack since buffer zone created

Announcing the demilitariazation agreement in September, Russian President Vladimir Putin and Turkish President Recep Tayyip Erdogan said the creation of a 15- to 20-kilometer (approximately nine to 12 miles) demilitarized zone would prevent a "humanitarian crisis" in the northwestern province.

However, shelling from the regime and the rebels has killed and injured dozens of civilians and militants, according to the SOHR.