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DAMASCUS, Aug. 2 (Xinhua) -- Syrian opposition activists and state-run media have traded accusations on Tuesday about possible, recent chemical attacks in northern Syria.

The Syrian Observatory for Human Rights said a shelling by barrel bombs filled with chlorine gas hit the town of Saraqeb in the northwestern province of Idlib on Monday evening, causing suffocation and breathing difficulties to 20 people.

The UK-based watchdog group said the type of gas was difficult to be identified, saying it has based its report on local testimonies.

The Syrian National Coalition opposition group cited activists as saying that it was the government forces who bombed the town of Saraqeb with chlorine-filled barrel bombs, placing the number of those affected at 30.

It urged the UN to take action, and said the alleged chemical attack took place in an area close to where a Russian helicopter was downed a day earlier.

On Monday, the rebels in Idlib countryside downed a Russian helicopter, with reports saying that all five crew members were killed.

Meanwhile, state-run SANA news agency blamed the rebels for firing a rocket containing poisonous gas on the government-controlled part of the northern city of Aleppo over the past 24 hours, killing five people, and eight others suffered suffocation.

SANA spelled no further details, but pan-Arab al-Mayadeen TV said that Moscow will investigate the terrorists' use of chemical weapons in Aleppo, and will also urge Washington to cooperate.

Chemical weapons are believed to have taken place in several areas in Syria in the past years, with the government and the rebels trading accusations.

As many as 1,400 people were killed when several opposition-controlled areas in the suburbs around Damascus were struck by rockets containing the chemical agent sarin on Aug. 21, 2013. Both the opposition and the government traded accusations.

In the same year, a chemical attack hit the then government-controlled town of Khan al-Asal in the countryside of Aleppo, in which several Syrian soldiers and civilians were either killed or suffered from suffocation. The government accused the rebels, who, in turn, denied the accusation.

In October 2013, the Organization for the Prohibition of Chemical Weapons (OPCW) officials arrived in Syria to monitor the dismantlement of the Syrian chemical weapons arsenal, after Damascus officially joined the Prohibition of Chemical Weapons Convention.

The OPCW later said that the government has made its chemical weapon production facilities inoperable.

The dismantlement of the Syrian chemical weapons was due to a U.S.-Russian understanding, the first sign of a consensus between both powers on the Syrian conflict.

Since then, reports of poisonous gas attacks kept emerging once in a while.