DAMASCUS, Aug. 21 (Xinhua) -- The Syrian army and allied troops backed by Russian air force are so close to lay the largest siege on the Islamic State (IS) militants in the Syrian desert in the remote eastern countryside of Homs province, a military source told Xinhua and activists said.

After recently capturing Al-Sukhnah city in eastern Homs from IS, the Syrian army was moving from another direction to form a circle around IS.

On Sunday, the Syrian army captured the town of Taibeh, north of Al-Sukhnah, becoming only 15 km to impose a full siege on around 9,000 sq km of IS-controlled terrain in the Syrian desert, the source told Xinhua on condition of anonymity.

The troops in Al-Sukhnah are advancing north of the city to meet with the troops who are closing in from Taibeh, said the source, adding that when the troops meet, the largest siege against IS will be in place.

So far, the IS militants have started fleeing through the remaining 15 km between Al-Sukhnah and Taibeh toward Deir al-Zour province in eastern Syria and the southern countryside of Hasakah province.

The months-long military operations in the Syrian desert by the Syrian army and allied fighters aimed to clear the desert from IS presence and to reach the administrative borders of Deir al-Zour to break the years-long IS siege on the city and the military forces trapped inside.

The operation is moving in that direction and breaking Deir al-Zour siege is now a priority for the Syrian army, the source told Xinhua.

In the south, near the capital Damascus, the Syrian army and the Lebanese Hezbollah group are fighting another battle against IS to clear the western Qalamoun region near Lebanon from the terror-designated group.

The source told Xinhua that the army and Hezbollah have captured key hilltops and heights, which will make it easy for the advancing troops to clear that area from IS, just a couple of weeks after the military forces and the Lebanese Shiite group defeated the al-Qaida-linked Nusra Front in the Qalamoun region.

Meanwhile, local media reported the falling of civilian victims as a result of the U.S.-led airstrikes on IS-held areas in the city of Raqqa, the de facto capital of the terror-labeled group.

At least 40 civilians have been killed over the past 24 hours by U.S.-led airstrikes on a building in Raqqa city, activists said, adding that all of the bodies are still under the rubble.

In Hasakah, at least 20 civilians were killed a day earlier when U.S.-led airstrikes heavily pounded areas in the eastern countryside of Hasakah, namely in the town of Jasa', activists said.

The U.S. airstrikes have escalated recently with the U.S.-led coalition backing the Kurdish-led Syrian Democratic Forces (SDF), which are advancing against IS inside the city of Raqqa.

The SDF said Monday that their fighters killed 50 IS militants in an area between Raqqa and Deir al-Zour.

The group posted a video footage, purporting to show large quantities of U.S. weapons and munition bound to SDF to support them in their battles of Raqqa.

The SDF said it has been advancing in the Mansura neighborhood in Raqqa.

Earlier this month, the Syrian Observatory for Human Rights said the SDF had captured over 55 percent of Raqqa from IS.

With the presence of IS is dwindling in Raqqa, the next big battle will be in Deir al-Zour, particularly that all the IS militants who are fleeing the desert and Raqqa are heading toward Deir al-Zour, an oil-rich province near the Iraqi border.