Islamic State (ISIS/ISIL) jihadis in northern Syria have launched a campaign targeting the Kurdish-led Syrian Democratic Forces (SDF), an American ally, a U.K.-based monitor group warned on Tuesday.

Citing the Syrian Observatory for Human Rights (SOHR), which uses a network of sources on the ground to monitor the conflict, the Agency France-Presse (AFP) news agency described weekend clashes between ISIS and the SDF as “counter-attacks” mounted by ISIS “from their embattled holdout in eastern Syria.”

According to the Observatory, ISIS came out of its last stronghold on Friday to attack a region in northern Syria’s Deir Ezzor province near the Iraqi border where SDF fighters and U.S.-led coalition advisers are based.

The ISIS “counter-attacks targeted the villages of Al-Bahra and Gharanij and an area close to the Al-Tanak oilfield, which is commercially active but is also an SDF military position,” AFP noted.

SOHR revealed on Monday that SDF fatalities from the fight “rose to at least 92” while the deaths suffered by ISIS reached 61.

The Observatory added that the ISIS attack on the SDF, the subsequent U.S.-led coalition airstrikes, and “booby-trapped vehicle explosions” reportedly killed at least 51 civilians, including 19 children and 12 women, over the weekend.

On Tuesday, the monitor group noted that ISIS cells continued to target the Kurdish People’s Protection Units (YPG)-led SDF forces.

Maj. Gen. Christopher Ghika, the deputy commanding general of the U.S.-led coalition’s mission against ISIS, declared in a statement issued on Sunday, “The presence of one ISIS terrorist anywhere is too many, but at this point in the operation, there are not enough ISIS to make any significant or lasting gains.”

The U.S.-led coalition reported that U.S.-backed Kurdish forces had “repelled a coordinated attack by ISIS” in Deir Ezzor province, adding:

[U.S.-led coalition] forces are employing combined fires, including indirect fire and close air support in support of Syrian Democratic Force partners, in order to contain ISIS in the last remaining territory they currently hold. Additionally, units from the Iraqi Security Forces are coordinating their efforts by defending the shared border with Syria to prevent ISIS forces from escaping back into Iraq.

ISIS took advantage of inclement weather and navigated among civilian populations as cover in an attempt to break out of their fighting positions within the MERV [Middle Euphrates River Valley].

The Observatory noted that since September 10 alone, ISIS has lost 739 jihadists, the SDF has suffered 452 fatalities, and clashes have killed at least 284 civilians, including 101 children and 69 women in northern Syria.

Referring to the latest clashes, the monitor group noted:

The Syrian Observatory monitored a state of discontent in the ranks of the [U.S.-backed] military forces located in the vicinity of ISIS enclave on the eastern banks of Euphrates River, where reliable sources confirmed that resentment came in the wake of the attack that took place after “the Islamic State Organization” went around sites reinforced by the Syria Democratic Forces in larger numbers of fighters in the last days before the attack.

ISIS jihadis stormed the Kurdish-held territory in “the desert and closed the roads to the front lines, which caused SDF members to [withdraw] from their positions on the front lines, fearing that they would be trapped or killed by the [jihadi] organization,” SOHR noted.

Some SDF fighters who fled the battle reportedly “took refuge in the desert, and some of them stayed there for more than 36 hours,” the Observatory learned from unnamed sources.

“More than 500” ISIS jihadis attacked the U.S.-backed fighters, mainly using “motorcycles carrying medium and heavy machine guns, followed by armored vehicles.”

The Observatory revealed the SDF has deployed reinforcement, numbering more than 1,000 fighters.

This month, the office of the inspector general at the U.S. Department of Defense (DOD), reported, “ISIS has lost all territory it held in Iraq and remained in control of only one percent of [the] territory it once held in Syria.” However, the DoD and a report produced by a United Nations Security Council monitoring committee stated that an effective clandestine ISIS organization has moved underground and is acting as an insurgency in both countries.

In late October, ISIS killed at least 40 U.S.-backed Kurdish fighters in northern Syria and re-conquered several areas they lost earlier that month near the Iraqi border.

The U.S.-led coalition noted that an estimated 2,000 ISIS jihadis remain in northern Syria.

Since May 2018, the U.S.-led coalition has been working to defeat the final remnants of ISIS in Iraq and Syria under Operation Roundup.