BEIRUT -- Syrian troops and allied militia have pushed back Islamic State militants and U.S-backed opposition fighters, gaining control of a large swath of territory in the country's strategic southern desert, the government-controlled media and a war monitor said Saturday.
Also Saturday, the U.S.-led coalition leading the campaign against the Islamic State group in Syria's northeast, acknowledged it has conducted airstrikes near the IS-controlled town of Mayadeen earlier in the week, targeting the group's "propaganda facilities." In a statement emailed to The Associated Press, the U.S.-led coalition said it is looking into reports the airstrikes killed over two dozen civilians, but added the claims were "unsubstantiated" and lacked "specificity or evidence."
With the new advances, the government and allied troops secured an area nearly half the size of neighboring Lebanon. The strategic juncture in the Syrian desert aids government plans to go after IS in Deir el-Zour, one of the militants' last major stronghold in Syria. The oil-rich province straddles the border with Iraq and is the group's last gate to the outside world.
The government and its allies have restored government control over mineral and oil resources, including the phosphate mines in Khneifes, once controlled by IS.
The state-controlled Syrian Central Military media said the new advances secured over 5,000 square kilometers (3,100 square miles) in the desert area, widening the government's control south of Palmyra in Homs province and the highway linking the ancient city to the capital Damascus.
The area was the backyard of territories once controlled by IS militants -- linking Palmyra, the Jordanian border area, the IS de-facto capital Raqqa, and the oil-rich Deir el-Zour.
The large swath of desert, parts of which were in rebel hands, also abuts Damascus and its suburbs. With their new gains, the government and allied forces have successfully isolated anti-government rebel fighters in the desert area east of Damascus, denying them advances toward the strategic Homs desert area.
The multi-pronged offensive has been ongoing for over two weeks and caused tension in the area, prompting a U.S. airstrike on Syrian government and allied troops near the border with Jordan.
The opposition Britain-based Syrian Observatory for Human Rights said the government and allied troops seized Saturday al-Ilianiya, an area controlled by Syrian opposition fighters, backed by the U.S. and western countries, in the desert near the border with Jordan. The Observatory said the government seized over 6,000 kilometers (3,700 miles) since the offensive began earlier this month.
The advances also pave the way for the government and its allied troops to press toward the IS-stronghold of Sukhna, a key node in the push toward the province of Deir el-Zour, said Mozahem al-Salloum, of the activist-run Hammurabi Justice News network that tracks developments in eastern Syria.
The crowded battlefield has been a scene of escalating friction in recent weeks, as the government and its allies pushed their way further south where rebel fighters backed by the U.S. military also operate. U.S. warplanes struck a convoy and a base of Syrian and allied troops on May 18, in the first such battlefield confrontation between American and Syrian forces since the conflict began in 2011.
U.S. officials said the Syrian advances posed a "threat" to its troops and allies fighting the Islamic State group in the area.
Meanwhile in Damascus, the Syrian government blamed the U.S.-led coalition for airstrikes that killed 35 civilians late Thursday in Mayadeen in Deir el-Zour province. The Syrian government called on the U.N. to condemn the airstrikes, saying they are "illegitimate" and should come to a halt.
Activists said those killed in the late night airstrikes included family members of IS fighters.
"We are aware of the unsubstantiated allegations of civilian casualties resulting from these strikes, however, many of the reports lack specificity or evidence," the U.S.-led coalition said in its statement to the AP. It said it is assessing the allegations as part of their routine review of all such claims.
The coalition said its airstrikes on Thursday and Friday targeted IS media facilities, "the number one driver of (IS)-inspired external attacks" around the world.
"(The) destruction of these ISIS targets degrades their ability to project terror," the statement said, using a different acronym for the Islamic State group.
Activists have reported that IS members fleeing Raqqa and Mosul, in Iraq, have found refuge in Mayadeen -- the town some say could potentially become the new IS capital as Raqqa comes under attack.
As the battle against IS picks up in Syria and Iraq, reports of deaths among civilians have been on the rise.
There are concerns that civilians in the last IS strongholds are prevented from leaving and could be subjected to retaliatory measures by the militant groups.
Associated Press writer Albert Aji contributed in Damascus, Syria, contributed to this report.
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