US-backed Syrian fighters have paused their offensive near the Tabqa dam so engineers can do any work necessary to ensure it continues to function.
The Syrian Democratic Forces alliance is battling so-called Islamic State for control of the structure, which is on the River Euphrates west of Raqqa.
On Sunday, the US-led coalition against IS denied claims by the group that air strikes have damaged the dam.
But its former chief engineer warned the control room was “out of service”.
The UN has warned that if the dam were to collapse, it could lead to flooding on a “massive scale” across Raqqa province and as far downstream as Deir al-Zour with “catastrophic humanitarian consequences”.
Why is the Tabqa dam important?
About 60m (200ft) tall and stretching 4.5km (2.8 miles) across the eastern end of Lake Assad, the dam is Syria’s largest.
IS captured the dam in 2014, giving it control of a vital reservoir and a hydroelectric power station that supplies large parts of the country.
The coalition also says the dam has been used by hundreds of IS foreign fighters as a headquarters, as a prison for high-profile hostages, as a training location and to plot attacks outside Syria.
Senior militants are said to have chosen to shelter at the dam because they believed the coalition would never bomb it and risk causing a flood.
The assault on the dam is part of a wider SDF offensive aimed at also driving IS militants from the nearby town of Tabqa and its airbase, which fell on Sunday.BBC.