Damascus:  Islamic State (IS) militants killed at least 70 Kurdish-led, US-backed Syrian Democratic Forces (SDF) fighters in a series of counterattacks in eastern Syria near the Iraqi border, a war monitor said.

The IS counter-offensive against the SDF has started on Friday and continued till Saturday, during which the terror-designated group recaptured areas it had lost to the SDF during an ongoing military showdown in the last IS-held pocket in the eastern countryside of Deir al-Zour in eastern Syria, Xinhua reported.

The Syrian Observatory for Human Rights said the warplanes of the US-led coalition, which is backing the SDF in its push to defeat IS on the eastern bank of Euphrates River, could not support the SDF in confronting the IS attack due to the bad weather conditions.

The Britain-based watchdog group said around 4,000 IS militants are located in a few towns in eastern Deir al-Zour near the Iraqi border.

The IS counter-attack targeted areas the SDF have recently captured in the towns of Souseh and Baghous, both located in the last IS-held pocket on the eastern bank of Euphrates River.

The IS recaptured large swathes of Souseh and Baghous, said the observatory.

The IS started its offensive by sending suicide car bombs to target the SDF positions, taking advantage also of the bad weather.

It was the most violent attack by IS against the SDF since the beginning of 2017, said the observatory, adding that tens of the SDF fighters had gone missing during the IS attack.

The SDF has started its operation in eastern Deir al-Zour on September 10 with the help of the US-led coalition.

The defensive tactics of the IS, such as landmines and suicide bombings, and the bad weather conditions, have contributed to slowing down the operation despite the fact that the Kurdish-led SDF managed to capture several positions from the IS in Hajin and the town of Souseh.

The Syrian army has cleared IS militants from large swathes of areas in Deir al-Zour, while the SDF is attempting to consolidate positions in the strategic oil-rich northern and northeastern countryside of the province near the Iraqi border.

The Syrian government has for long accused the US-led coalition of supporting IS under the table while pretending to be fighting it publicly.

The government has even accused the United States of committing massacres in Deir al-Zour, where 63 people have been killed this week as a result of the coalition’s airstrikes on IS-held areas in Deir al-Zour.