In the first alleged attack, cylinders suspected of containing chlorine gas were dropped in residential areas in the city of Saraqeb in Idlib province, the anti-regime, voluntary search-and-rescue group Idlib Civil Defence said in a post on its Facebook page.
Russia, Syria's key military backer, has vehemently denied that a chemical attack took place. In a news release carried by Russian and Syrian state news agencies, Kremlin spokesman Dmitry Peskov said the claims of chlorine usage were fabricated by the media.
The United States is not ready to officially call the incident in Saraqeb a chemical attack, but there is growing confidence that is what happened, a senior U.S. official told CNN on Tuesday evening. U.S. officials think chlorine was likely used, the source said. Evidence points to the Assad regime because the attack came from the sky and the opposition doesn't have any aircraft, the source added.
In the second alleged incident, the Syrian government claimed that "terrorist groups" carried out a gas attack that killed five people in the old town of the besieged city of Aleppo on Tuesday afternoon, according to the state-run news agency SANA.
"Five civilians were killed and eight others suffered suffocation due to a terrorist attack with shells containing poisonous gas," the city's health director Mohamad Hazouri told SANA, which reported two others were killed and 17 others injured in separate terrorist attacks in the city Tuesday.
Aleppo is about 50 kilometers (31 miles) northeast of Saraqeb.
Reports of the alleged chemical attacks came as a CNN stringer in rebel-held eastern Aleppo said intense airstrikes and artillery assaults were underway in the area, the fiercest bombardment in the five years of the conflict, he said.
The assault was a counteroffensive from Syrian regime forces and their Russian allies after advances by rebel forces pushing to break the government siege that has been choking their territory. The city's eastern neighborhoods were encircled by government troops last week, cutting off all supply lines.
In the Saraqeb attack, the gas, which the statement said smelled like chlorine, caused 30 cases of breathing difficulties, said the Idlib Civil Defence, also known as the White Helmets.
CNN has spoken with a doctor in Saraqeb who said he treated some of those affected by the alleged attack. He said their symptoms were consistent with those "of someone who has suffered from chlorine poisoning." A photographer who took photos of the injured for the White Helmets said victims were suffering symptoms such as watering eyes, spasms, sweating, coughing and difficulty breathing.
The Syrian National Council, a coalition of opposition groups, blamed the Assad regime for the alleged attack in Idlib province, where a Russian helicopter had earlier been downed after making what the Kremlin said was an aid delivery to Aleppo.
"Developments on the ground show how the Assad regime continues to blatantly ignore international law and U.N. Security Council resolutions, especially those related to the use of chemical weapons," the Syrian National Council said.
Posted by Women Against Shariah on Tuesday, August 2, 2016