US-backed forces in Syria are resuming an offensive against ISIL fighters at a major hydroelectric dam, saying it is in no danger of collapsing amid conflicting claims over its integrity.

The Kurdish-led Syrian Democratic Forces' (SDF) offensive was briefly suspended on Monday, a day after a senior Syrian government official warned that the Tabqa dam had been damaged by US-led air raids and cited an increasing risk of catastrophic flooding.

The Islamic State of Iraq and Levant (ISIL) group had also issued warnings that the dam could collapse "at any moment", releasing pictures showing what it said was the structure's control room after it had been damaged by US air raids.

THE SIGNIFICANCE OF TABQA DAM: The Taqba dam, also known as the Euphrates dam, is seen as a prize to push ISIL out of Raqqa, the armed group’s self-proclaimed capital in Syria. Lying about 40km from Raqqa, the dam is the biggest on the Euphrates. It stretches four kilometres across the river and is one of the few land crossings left as many bridges have been destroyed by fighting. But the dam has also been damaged. Last month, the UN warned that if it collapses there could be massive flooding across Raqqa and as far away as Deir Az-Zor, 150km downstream. The UN estimates about 90,000 people are "trapped" in Deir Az-Zor by ISIL.

The SDF, an alliance of Arab and Kurdish fighters, paused operations for four hours on Monday to allow engineers to inspect the dam, a major target in their campaign to encircle and capture ISIL's self-declared capital of Raqqa, located around 40km downstream on the Euphrates river to the east.

The SDF later said engineers had entered the dam and found no damage or "malfunction".

But it remains unclear whether engineers accessed the site.

The Syrian Observatory for Human Rights, a UK-based monitor tracking developments in Syria's conflict via a network of sources on the ground, denied the SDF's statement, according to news agencies' reports.

It said technicians inside ISIL-held Tabqa did not reach the dam during the ceasefire to reactivate its main power controls. There was no explanation given.

"If the dam had been breached, the United Nations was warning of a humanitarian disaster with thousands caught up in the flooding," Al Jazeera's Alan Fisher, reporting from Beirut, said.

"The SDF says there is no problem - the makeshift ceasefire is over and the battle for Raqqa a step closer."

READ MORE: The battle for Raqqa explained

The dam, which stretches for about four kilometres across the Euphrates, was captured by ISIL at the height of the armed group's expansion in Syria and Iraq in 2014.

The SDF's decision to briefly stop operations followed a request by the Syrian government's water authority, with officials blaming US air raids in the past two days for disrupting internal control systems and putting the dam out of service.

"There was growing concern over the weekend [about the dam's potential collapse]. In fact, ISIL was driving around nearby villages and towns warning them that there was a danger of flooding," said Al Jazeera's Fisher.

Hakam Tawfik, a structural engineer who worked on the construction of the dam, said it looked like the facility had been critically damaged.

"I've seen the pictures on the internet and the control and operations room had been fully burned, which means there is no control of the water coming into the dam," he told Al Jazeera from Remscheid, in Germany.

"This will lead to a real catastrophe, because there is no way from inside the dam to get rid of the water. The situation will escalate because as more cubic metres come in, the situation becomes more dangerous," Tawfik added, calling for an emergency operation.

"We need someone to interfere to stop this catastrophe, because water will overcome the dam and we don't know what we could do and how many days it would take to collapse - people living along there along the Euphrates are in danger."

The US-led coalition said on Monday that it saw no imminent danger to the dam, unless ISIL fighters planned to blow it up.

"We do not assess the dam to be in imminent danger unless ISIS plans to destroy it," said Colonel Joseph Scrocca, a spokesman for the coalition fighting ISIL, which is also known as ISIS.

"The SDF are in control of a spillway north of the dam that provides water to an irrigation reclamation canal which can be used to alleviate pressure on the dam if need be. If the lake reaches dangerous levels the SDF can relieve the pressure through alternative means."

Strategic airbase

Separately, the SDF announced on Sunday that it had captured the nearby Tabqa airbase from ISIL, a former Syrian military outpost that had been in the armed group's hands since 2014.

The allience said it had met pockets of resistance from ISIL fighters but it had managed to capture the strategically important base with the help of US-led raids.

READ MORE: Battle for Raqqa - 'Difficult and complicated'

Earlier this week, US forces airlifted SDF fighters and US advisers behind ISIL lines to allow them to launch the Tabqa assault.

"This is a part of the approach … the [US-led coalition's] assault on Raqqa, and gaining key strategic points, including the nearby airfield and the dam, all become part of the operation," said Fisher.

The SDF launched its offensive for Raqqa city in November, seizing around two thirds of the surrounding province, according to the Observatory.