DAMASCUS, Oct. 28 (Xinhua) -- The Islamic State (IS) militants have recaptured all areas lost to the U.S.-backed Syrian Democratic Forces (SDF) in eastern Syria, in its counter-offensive to retrieve these areas from the SDF since Friday, a war monitor reported Sunday.

It is a major setback for the SDF since the Kurdish-led forces launched an offensive against the last IS stronghold in the eastern province of Deir al-Zour on Sept. 10.

The Syrian Observatory for Human Rights said the SDF units seemed "not fully prepared to face the IS."

The recent counter-offensive claimed the lives of more than 70 SDF fighters, with over 100 others reportedly wounded, while the IS militants reached the Syrian-Iraqi border again, the London-based watchdog added.

The overall losses of the SDF fighters since Sept. 10 are estimated at moe than 300, according to the Observatory.

The SDF has brought in reinforcements to the IS-held pocket from forces involved in clearing the IS from its former de facto capital of Raqqa in northern Syria, the watchdog said.

Such forces could make a difference in the upcoming wave of attack against the IS despite the fortifications of IS and their tactics, it added.

In a previous report, the Observatory said bad weather conditions had prevented the U.S.-led coalition warplanes, which is backing the SDF in its push to defeat the IS on the eastern bank of Euphrates River, from supporting the SDF in confronting the IS counter-attack.

Around 4,000 IS militants are located in a few towns in eastern Deir al-Zour near the Iraqi border.

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 U.S.-led coalition of supporting the IS under the table while pretending to be fighting it overtly.

The government has even accused the United States of committing massacres in Deir al-Zour, where 63 people were killed last week as a result of the U.S.-led airstrikes on IS-held areas in Deir al-Zour.