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AMMAN, Jordan — Russian jets intensified their raids on rebel-held towns and cities in Syria's northern Idlib province on Sunday night, a day after rebels shot down a Russian warplane and killed its pilot.

Civil defense sources said air raids struck the towns of Kafr Nubl and Maasran, as well as the cities of Saraqeb, Maarat al Numan and Idlib, and that several deaths and dozens of injuries were reported as rescuers dug through the rubble.

A hospital was hit in Maarat al Numan, witnesses and residents said, and at least five people were feared killed in another attack that damaged a residential building in Kafr Nubl.

Video recorded by rescuers showed babies on stretchers being rushed by civil defense workers out of the damaged hospital while other rescuers fought to extinguish a fire.

In Idlib city, the provincial capital, one witness said a five-story building was leveled and that at least fifteen people were feared dead.

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In December, the Syrian army alongside Iranian-backed militias and heavy Russian airpower launched a major offensive to take territory in Idlib province, the last and most-densely populated province still mainly under opposition control.

Russia's defense ministry and Syrian rebels said the Russian SU-25 jet was downed on Saturday in an area that has seen fierce fighting on the ground and heavy air strikes targeting rebel groups opposed to President Bashar al-Assad.

Emergency services search the rubble after air strikes reportedly hit homes in Idlib in Syria on Sunday. Omar Haj Kadour / AFP - Getty Images

Syrians who oppose Assad view Russia as an invading force and blame it for the deaths of thousands of civilians since Moscow joined the war on the Syrian government's side in 2015.

Russia's Defense Ministry denies aiming at civilians and says it targets only hardline Islamist militants in Syria.

Air strikes on Saturday after the downing of the jet killed at least 10 people, including children, in Khan al Subl near where the plane crashed, rescuers said. The bodies of a family of seven were pulled from rubble following another attack in the town of Maasran, a local civil defense body said.

"We are pulling bodies from under collapsed walls. The Russians are taking their revenge on civilians, many of whom were already displaced and had fled their homes from earlier bombardment," said Ahmad Hilal, a civil defense rescuer.

The Syrian army and its Iranian Shiite militias allies made a string of gains in the last week after capturing a major air base that brought them just 7.5 miles from Saraqeb, the first heavily populated city in Idlib within their reach.

They were pushing towards the main Damascus-Aleppo highway, the capture of which would cut rebel supply lines and open the door to an army advance into the heart of the province.

A rebel fighter takes a picture of a downed Russian fighter jet on Saturday. Omar Haj Kadour / AFP - Getty Images

On Sunday, rebels led by the militant Turkestan group waged a counter attack against the Syrian army and Iranian backed militias near the Abu Duhur air base in southern Idlib.

The bombardment from the sky and fears of revenge by advancing Syrian troops and Iranian-backed militias have led to an exodus of tens of thousands of civilians further north to the safety of makeshift camps on the Syrian side of the Turkish border.

Syrians have poured into Idlib at an accelerating rate over the last two years, forced to abandon their homes in other parts of Syria that the government and its foreign military allies have recaptured from rebels.

The United Nations and aid workers have warned of a humanitarian catastrophe if the fighting nears the province's most heavily populated areas, where some two to three million people live.