BEIRUT -- Insurgents led by an al-Qaida-linked group launched one of their widest offensives yet Tuesday against Syrian pro-government forces, which responded with heavy airstrikes targeting hospitals and first responders.
Pro-government media said the airstrikes targeted insurgent supply lines from the northwestern Idlib province, a stronghold of the al-Qaida-linked group.
Opposition activists and rescue workers said at least two civilians were killed. They said two hospitals and two centers used by volunteer first responders known as the White Helmets were rendered nonoperational.
The offensive tests a newly announced "de-escalation zone" in Idlib, the latest of four such areas declared by Russia and Iran, which support the Syrian government, and Turkey, which backs the rebels. Fighting has largely subsided in the other three zones near Damascus, in the central Homs province and in southern Syria.
The Turkish Foreign Ministry announced last week that Iran, Russia and Turkey had agreed to deploy armed observers on the edge of Idlib, where troops would provide a cordon with checkpoints and watchtowers. It was unclear when these forces would be deployed.
The "de-escalation zones" have freed up Syrian troops and allied militias to battle the Islamic State group in the eastern Deir el-Zour province and other areas.
Hayat Tahrir al-Sham, a former al-Qaida affiliate, rejected the talks, which took place over months in the Kazakh capital of Astana. The group denounced the negotiations as a "betrayal" and a plot to hand opposition areas over to the Syrian government.
The Syrian government-affiliated Central Military Media outlet acknowledged the insurgent offensive just south of Idlib and said forces responded with intensive airstrikes and shelling, killing a number of insurgents.
Rami Abdurrahman, the head of the Britain-based Syrian Observatory for Human Rights, said airstrikes pummeled the front line but also targeted southern Idlib, hitting at least four medical centers.
The opposition-run Qasioun News Agency said two hospitals were knocked out of service because of the strikes. The White Helmets, also known as the Syrian Civil Defense, said two of its centers were also nonoperational. The group counted at least 40 airstrikes in the area since early Tuesday.
One medic was reported killed in an airstrike on al-Tih hospital in southern Idlib, according to activists.
Abdurrahman said the offensive is the widest so far targeting the government-held city of Hama. He said thousands of fighters, led by Hayat Tahrir al-Sham and its allies, including Chinese fighters of the Turkistan Islamic Party, took part in the offensive.
Also on Tuesday, the U.S.-led coalition against the Islamic State group confirmed that it had closed one of its two bases in southern Syria. Coalition spokesman Col. Ryan Dillon said that after the closure of the Zakaf base, coalition and allied Syrian forces are now only operating out of the larger Tanf base near the Jordanian and Iraqi borders.
Information for this article was contributed by Sarah el Deeb and Vladimir Isachenkov of The Associated Press; and by Erin Cunningham and staff members of The Washington Post.
A Section on 09/20/2017