"A regime helicopter was forced to land in the region of Jabal al-Zawiya in the northwest, which is a bastion of (Al-Qaeda's Syrian affiliate) Al-Nusra Front," Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, told AFP.
"Four of the crew were captured and a fifth man was killed by armed men in a neighbouring village," he said.
Abdel Rahman said the Al-Nusra Front held two of the servicemen, including the helicopter's pilot.
Official Al-Nusra Front accounts on Twitter published photos and videos with the caption "Helicopter pilot in the hands of jihadists."
The images depict the bandaged pilot sitting in front of Al-Nusra Front's official flag.
"To the Muslim children, to their mothers, who died two days ago because of their chlorine attack," a bearded fighter says in the video, pointing to the pilot.
On March 17, a family of six died in Idlib province after a regime gas attack on the village of Sarmin. Opposition forces accused the regime of using chlorine, a toxic agent that can be considered a chemical weapon.
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Two other servicemen from the helicopter crash were captured by another Islamist group, and at least one more crew member was believed to be on the run, Abdel Rahman said.
Regime helicopters are often used to drop crudely constructed barrel bombs on rebel-held areas.
Pictures provided by the Observatory showed groups of men gathering around a damaged helicopter lying on its side on a rocky hilltop.
Syrian state television confirmed a military helicopter had crashed in Idlib, saying it was due to a technical failure and that search efforts were underway to locate the crew.
In the southern province of Daraa, opposition fighters and forces loyal to Syrian President Bashar al-Assad clashed in the ancient town of Busra al-Sham, the Observatory said.
At least nine opposition fighters were killed, but the casualty toll on the regime side was not immediately known.
The town has both Sunni and Shiite Muslim residents, with control divided between regime and rebel forces.
Abdel Rahman said opposition groups shelled the Shiite neighbourhoods heavily on Sunday, a day after regime forces had shelled rebel-held areas.
A regime helicopter also dropped at least one barrel bomb on the town Sunday, but Abdel Rahman had no immediate report on civilian casualties.