Syrian rebels have accused the government, of embattled Syrian President, Bashar al-Assad of using poison gas against civilians in the north, while the government has instead accused the rebels of using chemical weapons themselves.

Russia is also denying claims from Syrian rebels and human rights groups that it was behind an alleged chlorine gas attack in the same area where a Russian helicopter was shot down the day before, according to a Russian state media report.

“I don’t have information; we don’t have information about how the operation is proceeding,” Russia’s TASS news agency reported Kremlin spokesman Dmitry Peskov as saying. “It is very hard to react to such information releases: it is not always clear what they are based on, what their sources are and so on.”

Aid workers in Syria’s war-ravaged northwest claimed that a helicopter dropped barrels of toxic gas on a town near the besieged city of Aleppo.

There has been no independent confirmation of any claim, however, the Russian military, as well as Russian news agency Interfax, has alleged that rebels in the Syrian city of Aleppo deployed the “toxic substances.”

Like Interfax, Syria’s state-run SANA news agency also reported that rebels were behind the attack in Aleppo. SANA’s report added that Mohamad Hazouri, Aleppo’s health director, put the death toll at eight and said victims reported breathing problems from the chlorine shells.

Russian Foreign Ministry official, Mikhail Ulyanov said he has appealed the chemical strikes to the Organization for the Prohibition of Chemical Weapons.

“We have accurate information that [rebels have] the relevant knowledge and even industrial capacity to produce chemical warfare agents,” Ulyanov said

Syrian neurologist Ibrahim al-Assad said he treated 16 of the 29 cases brought to his hospital in the rebel-held part of Syria.

Doctor al-Assad said first responders smelled the gas at the site of the bomb attack, which he described as a busy shopping area near an ice cream shop. Rebels and activists have reported chlorine gas attacks in the town before, but the lack of chemical labs or independent testers makes it difficult to verify these claims.

The head of the Russian military’s Reconciliation Center, at the Russian base in Syria, Lt. Gen. Sergei Chvarkov, said in a statement that the rebels struck a residential area in Aleppo with toxic substances, killing 7 and injuring 23. He said the Russian military had informed the U.S. about the incident.

The Organization for the Prohibition of Chemical Weapons, OPCW, said in a statement that recent media reports that highlighted the possible use of chemical weapons in Syria are of great concern. It said the agency continues to examine any credible reports it receives regarding responsibility for the attacks, including pertinent information that might be shared by state parties to the Chemical Weapons Convention.

The Syrian government has repeatedly been accused of attacking rebels with crude chemical bombs.

Earlier this year, the Syrian-American Medical Society (SAMS) said chemical weapon attacks have killed nearly 1,500 people since civil war broke out in Syria more than five years ago. The SAMS report, released in March, documented 161 chemical attacks in Syria and linked the vast majority of them, and the resulting civilian casualties, to the Assad government.

Meanwhile, a major Syrian opposition announced that the government siege had finally been broken last week by rebels. Al Manar, the television station affiliated with the armed Lebanese group Hezbollah, which is fighting alongside the Syrian government forces, said “the rumours that the siege on the eastern neighbourhoods has been broken is entirely false” in a statement on its website.

It is expected to be decisive for the future of a city that was once a commercial heartland, but has been destroyed by five years of war.

About a quarter of a million civilians are living under siege since government forces cut them off last month. In the western, government-controlled areas, UNICEF said 25,000 people have been displaced and are taking shelter from intense fighting in mosques, university campuses, and public gardens.

The latest reports from the area are that the government, backed by Russian jets, has regained some ground. It is not clear exactly which territory has returned to government control. Russian and Syrian forces say they have been operating seven so-called humanitarian corridors, allowing hundreds of people to leave the besieged area peacefully. Russian state television aired pictures of civilians and fighters leaving as gunshots were fired and smoke was seen billowing over the city in the footage.

UNICEF’s regional director, Saad Houry said he is “extremely” concerned for the safety of children caught up in violence in the city, as they make up a third of the 300,000 residents trapped in besieged areas, and called for unhindered humanitarian access and for children to be protected.