USA -backed Syrian Kurdish forces were in control of a spillway north of the dam "which can be used to alleviate pressure on the dam if need be", the coalition said in a letter to The Associated Press.

Syrian forces now control territory up to 13 miles around the historic city of Palmyra, Rudskoi said, adding that they recently seized control of high ground held by the Islamic State along a highway connecting Palmyra and the Syrian capital, Damascus.

A coalition operation led by the United States dropped off a contingent of Syrian opposition forces behind the Islamic State of Iraq and the Levant (ISIL) lines in what US officials called a daring raid to cut off ISIL forces from one of the last remaining major supply lines to Raqqa.

USA -backed forces in northern Syria said Monday they had paused military operations near a major dam held by the Islamic State group in order to allow engineers to fix any problems after conflicting reports about its stability.

"Islamic State will put up fierce resistance, especially since the dam is strategic", she said, but the dam "will be liberated in a short period of time".

Rami Abdulrahman, the chief of the United Kingdom -based Syrian Observatory for Human Rights (SOHR), said ISIS fighters told residents Sunday that the Tabqa dam had collapsed.

The fighting stopped only so that technicians could enter the dam site to ensure it was sound.

This undated picture provided by Syria's official news agency SANA shows the aftermath of a double bomb attack in the western Syrian city of Homs. "The people were scared that the dam had stopped".

Syrian farmers near the Euphrates say they are terrified IS will blow up the dam to defend Raqa, drowning their tiny villages in the process. The activist group Raqqa is Being Slaughtered Silently reported that ISIS had ordered Raqqa residents to evacuate, though without their furniture.

The extremist group proceeded to tell the civilians that the story was not true, forcing them to return.

The hills of Raqqa served as the backdrop to the group's many hostage beheading videos in 2014 and 2015, where monitors believe captives were taken to be killed.

The dam had not been structurally damaged, according to the statement, which added that the SDF now controls a spillway north of the dam that could reduce water pressure if it rose to risky levels. It was a move to ease pressure on the dam's reserves.

The United Nations warned this year of the risk of catastrophic flooding from the dam, Syria's largest, which is at risk from high water levels, deliberate sabotage by IS and further damage from air strikes by the USA -led coalition.

The SDF and coalition have denied this.

The director of the Syrian government's General Authority of Euphrates Dam, which formerly operated the huge project, blamed U.S. strikes in the past two days for disrupting internal control systems and putting the dam out of service.

Fox News is told the preliminary plans would have the Kurds surround Raqqa, and an Arab force would lead the assault into Raqqa.

The campaign is ultimately aimed at isolating the ISIS radical group in its de facto capital.

"The Tabqa airbase is a key position for the liberation of Raqqa city", Rafi, an SDF commander in the area told Syria Direct on Monday.