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BEIRUT — A U.S.-backed Syrian fighting force said Tuesday it has begun the final stage of its campaign to clear Islamic State group militants from northeast Syria.

Separately, at least 21 pro-government fighters were killed in clashes with the militants in south Syria, underlining the persistent threat posed by the group despite its shrinking footprint.

The Britain-based Syrian Observatory for Human Rights monitoring group said the 21 fighters, among them Syrian soldiers and foreign militiamen who have been recruited to fight for President Bashar Assad, died Monday night in a battle with IS militants in the Safaa desert region at the edges of Sweida province.

The activist-run pro-government Facebook page Damascus Now however reported that only 14 fighters died in the fighting. Such discrepancies on casualty figures are not uncommon in the immediate aftermath of the fighting.

Government forces have been battling the militants in the area for weeks since IS fighters ambushed Sweida in July.

The militants are believed to be holding some 30 Sweida civilians captive in the Safaa area since kidnapping them in the July ambush, when some 300 people were killed during the daylong battle.

In north Syria, the Syrian Democratic Forces said it launched operations to expel IS militants from the town of Hajin and surrounding villages on the northeast banks of the Euphrates River.

The SDF is backed by the U.S.-led international coalition against IS.

The coalition said in a statement it was supporting the SDF ground operation with air and artillery strikes.

The Observatory said coalition jets have been bombing locations across the IS pocket in conjunction with the SDF ground offensive. At least 23 IS fighters were killed in the first 24 hours of battle, according to the Observatory.

Despite losing 90 percent of its territory between Iraq and Syria since its 2014 peak, the jihadist group remains a disruptive force in both countries. Its leader, the self-styled "Caliph" Abu Bakr al-Baghdadi, urged followers to "persevere" in an audio tape attributed to him last month.

Also Tuesday, Turkey's President Recep Tayyip Erdogan urged the international community to stop the Syrian government from embarking on a widely-expected campaign to retake Idlib province from rebels.

Idlib is the last refuge for the Syrian opposition and home to close to 3 million people, including 1.4 million displaced Syrians hailing from provinces now controlled by the government.

Erdogan, in an op-ed printed in the Wall Street Journal, said an assault on Idlib would "create serious humanitarian and security risks for Turkey, the rest of Europe and beyond."

The op-ed came as high-level diplomats from Turkey, Russia, and Iran met with the U.N. envoy for Syria about creating a committee to revise the war-battered country's constitution, in Geneva.

Russia, Turkey and Iran have been working together as "guarantors" for a series of talks around ending Syria's war. Turkey has taken in 3.5 million refugees from its neighbor.

Syrian government forces, backed by Russia and Iran, have been massing troops for weeks around Idlib in preparation for an attack.

Erdogan said an attack would amount to "indiscriminate attacks to wipe out its opposition and not a genuine or effective campaign against terrorism."

Turkey's has 12 military outposts inside Idlib province and supports various factions inside. In recent days, Turkey has moved dozens of armored vehicles across its border and into Idlib, the Observatory reported.

But Russia and Iran say they are determined to annihilate the al-Qaida-linked Hayat Tahrir al-Sham group that is the dominant fighting force in the province.

The Turkish president said Russia and Iran have the responsibility to stop the humanitarian disaster.

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Associated Press writers Jamey Keaten in Geneva and Suzan Fraser in Ankara, Turkey contributed to this report.