The United Nations is warning of catastrophic flooding in Syria from the Tabqa dam, which is at risk from high water levels, deliberate sabotage by the Islamic State group and further damage from air strikes by the US-led coalition.
The earth-filled dam holds back the Euphrates River 40 km upstream of the IS stronghold of Raqqa and has been controlled by IS since 2014.
Water levels on the river have risen by about 10 metres since January 24, due partly to heavy rainfall and snow and partly to IS opening three turbines of the dam, flooding riverside areas downstream, according to a UN report seen by Reuters on Wednesday.
"As per local experts, any further rise of the water level would submerge huge swathes of agricultural land along the river and could potentially damage the Tabqa Dam, which would have catastrophic humanitarian implications in all areas downstream," it said.
The entrance to the dam was already damaged by airstrikes by the US-led coalition, it said.
"For example, on 16 January 2017, airstrikes on the western countryside of Ar-Raqqa impacted the entrance of the Euphrates Dam, which, if further damaged, could lead to massive scale flooding across Ar-Raqqa and as far away as Deir-ez-Zor."
The town of Deir-ez-Zor, or Deir al-Zor, is a further 140 km downstream from Raqqa, and is besieged by IS. The UN estimates that 93,500 civilians are trapped in the town, and has been airdropping food to them for a year.
The US-backed Syrian Democratic Forces (SDF) are undertaking a multiphased operation to encircle Raqqa, and have advanced to within a few kilometres of the dam. The SDF has previously said air strikes are not being used against IS near the dam to avoid damaging it.
As IS retreats, its fighters have deliberately destroyed vital infrastructure, including three water stations and five water towers in the first three weeks of January, the UN report said.
"IS has reportedly mined water pumping stations on the Euphrates River which hinders the pumping of water and residents are resorting to untreated water from the Euphrates River."
The UN has also warned of the danger of a collapse of the Mosul dam on the Tigris River in Iraq, which could affect 20 million people. The dam was briefly captured by IS in 2014, but remains at risk, with constant repairs needed to avoid disaster.
Last month Lise Grande, the top UN humanitarian official in Iraq and a trained hydrologist who is an expert on the Mosul dam, said a catastrophic burst could have "Biblical" consequences. The UN is preparing an international response in case the Mosul dam collapses
