Syrian engineers were expected to carry out urgent maintenance Wednesday on the countrys largest dam, where US-backed fighters have been battling Islamic State group jihadists who still control most of it.

The maintenance work, which entails opening a spillway to relieve the pressure of water on the dam, will involve the engineers entering areas held by IS, a technician inside the complex told AFP.

The fighting between the US-backed Syrian Democratic Forces and IS has raised fears for the integrity of Tabqa Dam, which holds back a vast reservoir in the Euphrates Valley that could cause catastrophic flooding if it burst.

The technician inside the complex said that rising water levels in recent days had submerged some equipment in its lower levels.

"Because the dam has been out of service for three straight days, the technical equipment in the lower levels of the dam is under water," the technician said.

"This rise in the level of the reservoir means that one of the spillways must be opened to drain the water so it doesn't build up, which would pose a growing threat to the dam."

Both the SDF and its US-led coalition backers have denied any kind of "structural damage" to the dam.

But the technician inside the complex and the Syrian Observatory for Human Rights said the dam's main control room had been knocked out.

The technician said engineers had arrived from SDF-controlled territory and would "try to enter the dam and carry out the necessary maintenance as quickly as possible, if they are allowed."

The SDF paused fighting for four hours on Monday to allow technicians to enter the complex and carry out maintenance.

Earlier this year, the United Nations raised concern about the risks of damage to the dam in fighting, warning that water levels in the reservoir were already high.

IS has accused the US-led coalition of bringing the dam to near-collapse with its air strikes, and on Tuesday it charged that a US strike had killed the dam's top technicians.

An AFP correspondent at the dam's northern entrance saw engineers examining part of the structure on Tuesday accompanied by members of the Syrian Arab Red Crescent.

"The explosions and the clashes are threatening the dam, and we ask for all sides to distance themselves from it," said Ismail Jassem, an engineer from the SDF-controlled Tishreen Dam upstream.

"The water levels are acceptable now. We came to open up one of the gates to relieve the pressure," he told AFP.

The SDF assault on the dam is part of a wider US-backed offensive to capture IS's de facto capital of Raqa downstream.

SDF fighters have advanced to within eight kilometres (five miles) of the city at their closest point but are between 18 and 29 kilometres (11 and 18 miles) away on other fronts.