Neither of the accusations could immediately be verified.
The Syrian state news agency SANA said "terrorists" fired a rocket "containing chemical materials" into the area around the village of Khan al-Assal in the northern province of Aleppo. The regime regularly uses the term terrorists to refer to rebels fighting to overthrow authoritarian President Bashar Assad.
SANA said about 15 people, most of them civilians, were killed and a number of others were wounded.
An activist in the area said rebels had recently seized much of the village of Khan al-Assal including a facility that housed a military academy.
The Aleppo Media Center, affiliated with the rebels, said there were cases of "suffocation and poison" among civilians in Khan al-Assal after a surface-to-surface missile was fired at the area. It said in a statement the cases were "most likely" caused by regime forces' use of "poisonous gases."
An activist in Aleppo province who identified himself as Yassin Abu Raed, not his real name, confirmed the attack and said there were at least 40 cases of suffocation in the area and several deaths. But he said no details were available as casualties were being taken to a government controlled area in Aleppo.
Abu Raed declined to give his real name because of security concerns.
He said it did not make sense for the rebels to fire a chemical weapon at an area they had recently seized, and accused the government instead.
"Why would the Free Syrian Army bomb themselves with a chemical weapon?" he asked.
There has been long-standing concerns that Syria's chemical weapons would be used by one side or the other in the 2-yearold civil war.
U.S. Secretary of State John Kerry said recently that the longer the war goes on, the greater the danger of its institutions collapsing and extremists getting their hands on the Arab country's vast chemical weapons arsenal.
The reported attack was in an area just east of the city of Aleppo that had seenfierce fighting for weeks before rebels took over a sprawling government complex there last month. The facility included several military posts and a police academy that Assad's forces have turned into a military base that regularly fires shells at nearby villages.