AMMAN: An explosion ripped through a residential neighborhood in Idlib province’s Jisr a-Shughour early Wednesday morning, killing more than a dozen civilians and sending massive plumes of black smoke over the city center.

Local rescue workers reported 15 dead and 30 wounded, including several children, by Wednesday afternoon.

“The source of the explosion is still unknown, but our preliminary information indicates that this was a booby-trapped [car] bomb,” said Ahmad Yazji, a member of the Idlib branch of the Syria Civil Defense group of first responders often known as the White Helmets.

According to Yazji, casualty numbers would likely rise as rescuers were still digging out bodies and searching for survivors among the debris on Wednesday afternoon.

All of the victims recorded so far were civilians, he told Syria Direct.

Wednesday’s bombing reportedly left extensive damage to residential buildings in the center of the western Idlib city.

However, according to the Syrian Observatory for Human Rights (SOHR) conflict monitor, the bombing targeted a local governance building belonging to hardline Islamist faction Hay’at Tahrir a-Sham (HTS), which controls the majority of Syria’s rebel-held northwest.

A city in the far west of Idlib’s western countryside, Jisr a-Shughour has been controlled by hardline Islamist factions including HTS—as well as the Al-Qaeda-affiliated Turkistan Islamic Party (TIP)—for several years.

While no group had claimed responsibility for the attack by Wednesday afternoon, the explosion comes amid a widespread security crackdown by HTS following a wave of attacks across northwestern Syria that many experts say bear the hallmarks of Islamic State (IS) sleeper cell operations.

Hours before, HTS launched a series of counter-insurgency operations in the southern Idlib countryside.

HTS-affiliated officials claimed the operations were targeting “sleeper cells of the Islamic State.”  

On Tuesday, HTS units carried out raids on several sites around the village of Mesaybin, engaging in a drawn-out gun battle that reportedly lasted for hours.

Ten suspected IS members were arrested in the wake of the shootout, according to an HTS-affiliated official.

The operations in southern Idlib mirror a larger anti-IS campaign that has taken shape in recent months across the northwest, as HTS and officials from the HTS-affiliated Syrian Salvation Government (SSG) grapple with what they call a widespread sleeper cell threat in the region.

Following a series of raids targeting sites linked to suspected IS sleeper cells, HTS militants publicly executed 10 individuals that it claimed were affiliated with IS on March 2.

More arrests of suspected IS commanders from the group continued throughout that same month.

While IS has lost all the territory it once controlled in the east of the country, analysts tell Syria Direct that thousands of sleeper cells have been lying dormant in the rebel-held northwest since as early as 2014.

The Syrian government also stands accused of using a series of evacuation deals last year to redirect surrendering IS fighters towards the northwest, and territories controlled by HTS.  

Increasingly, IS sleeper cells appear to have waged a bloody and destablizing campaign against their longstanding rivals in Idlib—including targeted assassinations against SSG officials, car bombings and hit-and-run-style attacks striking key infrastructure.

In January, a veiled woman wielding two machine guns entered the SSG headquarters in Idlib city and detonated herself in a suicide attack.

Civilians have also frequently been caught in the middle of the feud, with scores killed in a series of attacks around Idlib in recent months.

A twin bombing that shook the city in February killed at least 15 civilians, including a number of children.

A commander from HTS, speaking with Syria Direct on condition of anonymity earlier this week, was quick to blame IS sleeper cells for the growing wave of violence that risks further destabilizing Syria’s last remaining rebel enclave.

“IS sleeper cells are responsible for most of the chaos and assassinations sweeping the region,” he told Syria Direct on April 22. “We are currently preparing a campaign to eliminate these cells, and take control of security across the region.”

As smoke continued to rise from the wreckage of downtown Jisr a-Shughour in the hours after Wednesday morning’s attack, Syrian government warplanes launched bombing sorties just outside the city—striking targets around Jabal a-Sahel.

Dozens of civilians have also died in recent months amidst a dramatic uptick in pro-government shelling across the northwest.

Caught in the middle of a hardline Islamist power struggle and government bombardment are an estimated three million civilians currently residing in Idlib.Around a third of those civilians are from communities displaced northward by a series of pro-government offensives last year that saw much of the country return to Syrian government control—including several pockets of territory formerly controlled by IS.