Syrian government forces and their Russian allies struck two hospitals, an apartment building and several other targets in a rebel-held province, activists said Monday, amid relentless airstrikes following the downing of a Russian fighter jet in the area and the killing of its pilot by al-Qaida-linked militants.

The assault on Idlib, the last rebel-held province in Syria, intensified in recent weeks but reached a new ferocity after the militants shot down a Russian Su-25 near the town of Saraqeb on Saturday.

Russia has waged a punishing aerial campaign against Syria's armed opposition since intervening in the civil war on the side of its ally, Syrian President Bashar Assad, in 2015.

The al-Qaida-linked Levant Liberation Committee, which is the strongest militant group in Idlib, said its fighters downed the Russian jet and killed its pilot after he ejected from the plane and landed on the ground. Russia's Defense Ministry confirmed the downing, saying preliminary information indicated the plane was shot by a portable ground-to-air missile in an area under control of al-Qaida's branch in Syria.

On Sunday, a search-and-rescue group and a medical charity reported that several people suffered breathing difficulties after a suspected chlorine gas attack on Saraqeb, days after the Trump administration accused Assad's government of producing and using "new kinds of weapons" to deliver poisonous gases. Damascus denied the White House's charges, and The Associated Press could not independently verify the reports of a chlorine gas attack.

A U.N. investigative commission said in 2016 that it had determined the Syrian government was behind at least three chlorine gas attacks during the seven-year civil war, but activists and monitoring groups claim there have been more attacks.

The Syrian Civil Defense, also known as the White Helmets, said three of its rescuers and six other people suffered breathing problems. The Syrian American Medical Society, or SAMS, said its hospitals in Idlib treated 11 patients for chlorine gas poisoning.

On Monday morning, a hospital in Kafranbel, another town in Idlib provnce, was bombed, according to the activist-run Edlib Media Center and the Britain-based Syrian Observatory for Human Rights monitoring group.

Another hospital, in Maaret al-Numan, was struck three times on Sunday night and put out of service, according to SAMS, which runs the facility.

An apartment building in the city of Idlib, the provincial capital, was also destroyed, said the Observatory.

"It's just punishment," said Wissam Zarqa, a media activist in Idlib. "When you are targeting hospitals, targeting Idlib city, it's just to say 'I am here, and I can hurt you.'"

Last Tuesday, a hospital supported by Doctors Without Borders in Saraqeb was damaged in an airstrike, killing five people, according to the medical aid group.