Rebel fighters launch a locally made shell toward forces loyal to Syrian President Bashar Al-Assad, who are stationed in checkpoints surrounding the city of Idlib, on March 23. (Mohamad Bayoush/Reuters)

Syrian rebels launched an offensive Tuesday against a major government-held city in the country’s northwest, shelling the outskirts and warning residents to remain indoors.

The operation’s target is Idlib, a city of about 165,000 and the capital of the province of the same name. Rebels have controlled countryside and towns across the province since 2012, but President Bashar al-Assad’s forces have held Idlib city.

Activists said Syrian government helicopters attacked the nearby town of Binish with chlorine gas Tuesday night. The Local Coordination Committees activist collective did not give details on casualties, but Muayad Zurayk, an activist based in Idlib province, said 30 people were rushed to the hospital after experiencing breathing problems.

The alleged attack came a week after the opposition claimed that the government carried out a poisonous gas attack on the nearby town of Sarmin, killing six and wounding dozens. Damascus denied the allegation.

Armed opposition factions announced the campaign to capture Idlib in a message posted online Tuesday. They told residents that the rebels “are at the walls of Idlib” and “have decided to liberate this good town.” The message also asked locals to remain indoors in the coming days.

A rebel fighter carries his weapon as he walks along orchards near the frontline of Idlib city in northern Syria on March 22. (Khalil Ashawi/Reuters)

Syrian state TV quoted an unnamed military official as saying that government forces are repelling “attempts by terrorist groups to infiltrate the outskirts” of Idlib. The official said clashes were ongoing, adding that troops inflicted “heavy losses” on the attackers. The government refers to the rebels as terrorists.

Zurayk, the activist based in Idlib province, said the offensive began Tuesday morning and was being led by several rebel factions including Jabhat al-Nusra, the al-Qaeda branch in Syria.

— Associated Press