BEIRUT -- The Islamic State group killed at least 40 U.S.-backed Syrian fighters, captured several alive and regained areas they lost earlier this month in eastern Syria near the Iraqi border in some of the most intense fighting in weeks, a war monitor and an agency linked to IS said Saturday.

Members of the U.S.-backed Syrian Democratic Forces have been on the offensive since early September under the cover of airstrikes by the U.S.-led coalition to capture the last pocket held by IS in Syria.

Friday's fighting that lasted until the early hours of Saturday began when IS, taking advantage of a sandstorm, launched a counteroffensive against SDF positions east of the Euphrates river in the eastern province of Deir el-Zour that borders Iraq, activists said.

Col. Sean Ryan, spokesman for the U.S.-led coalition, said they have no confirmation of exact figures "as both sides are taking casualties." He called it a "difficult fight."

"The sandstorm allowed an ISIS counterattack, which was surprising given the conditions, but now the air is clear and the Coalition will continue to increase air and fire support to assist our partners," Ryan said using a term to refer to IS.

Rami Abdurrahman who heads the Britain-based Syrian Observatory for Human Rights said that since Friday IS has killed more than 60 SDF fighters, wounded others and captured at least 20. He added that some 100 SDF fighters have fled the battlefield as the extremist group has carried out suicide car bomb attacks.

The Observatory and the Deir Ezzor 24 activist collective said IS fighters captured the village of Sousa that they had lost control of earlier this week.

The IS-linked Aamaq news agency said that more than 40 SDF fighters were killed and posted a video of six gunmen captured alive.

An SDF official did not immediately respond to inquiries sent by The Associated Press.

The Observatory said that the fighting continued until early Saturday and that IS gunmen attacked SDF positions on the eastern banks of the Euphrates river in east Syria.

The last pocket held by IS in Syria is home to thousands of civilians caught in the crossfire with nowhere else to go.

Brett McGurk, the White House envoy for the war against IS, told a conference in Bahrain on Saturday that "the military mission in Syria is the enduring defeat of ISIS." He said he expected IS would be defeated over the coming months.

"It is very difficult because we are in the last stages, where almost every ISIS fighter is a suicide belt," McGurk said. "It's very difficult fighting but we will get it done."

Last week, U.N. humanitarian official, Jan Egeland said in Geneva that some 15,000-people including IS fighters and their families live in the area.

"A lot of civilians there are being attacked from all sides," Egeland said.

McGurk said that the U.S. and the SDF are holding some 700 foreign fighters in Syria, adding that they are determined that "these people cannot get out of Syria to threaten our homelands or yours." He said the coalition is working with some countries to make sure that "we could try to repatriate these people."

While the process is underway, McGurk added, the U.S. is working to ensure that they are housed in a facility inside Syria that meets international standards "so that these people cannot commingle and repeat any of the problems that we've seen in the past."

SDF spokesman Kino Gabriel told the Kurdish ANHA news agency on Friday that an international tribunal should be set up to try the foreign IS fighters his group is holding. He said that the SDF has apprehended more than 700 non-Syrian IS fighters in addition to their families.

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Youssef reported from Cairo.

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