Syria's Tabqa, which US-backed forces entered Monday, is doubly strategic: it lies on the road to the Islamic State group's Raqa bastion and the nearby dam is the country's largest.

Tabqa is both the name of the town and the dam and capturing both would allow the US-backed Arab-Kurdish alliance known as the Syrian Democratic Forces (SDF) to advance on Raqa.

The de facto Syrian capital of IS's self-proclaimed "caliphate" is about 55 kilometres (34 miles) to the east of Tabqa in northern Raqa province.

On Monday the SDF, who are supported by US-led coalition air strikes and special forces advisers on the ground, penetrated Tabqa, overrunning territory in the town's south.

Tabqa's population has dwindled from 250,000 before Syria's conflict erupted in 2011 to 75,000.

In addition around 10,000 jihadists and their families who have come from Arab countries, Europe, Australia and the United States now live in Tabqa, which serves as an important IS command base and houses the group's main prison.

The dam is built on the 2,800-kilometre-long (more than 1,700-mile-long) Euphrates River, which flows from Turkey through northern Syria and east into Iraq.

It is 4.5 kilometres (2.8 miles) long, 60 metres (about 200 feet) high and 512 metres (1,680 feet) wide at its base.

- Out of service  - 

Its reservoir, Lake Assad, stretches along 50 kilometres (30 miles) and covers a surface of 630 square kilometres (240 square miles). Its total capacity is 12 billion cubic metres (around 420 billion cubic feet) of water, making it Syria's main reserve.

The dam fell into the hands of Syrian rebels in February 2013, before IS seized control of Raqa and its eponymous province in early 2014.

The facility went out of service in late March after bomb damage to its power station, risking rising water levels.

The UN's humanitarian coordination agency OCHA warned at the time that damage to the dam "could lead to massive scale flooding across Raqa and as far away as Deir Ezzor", a province downstream.

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.

- Soviet help -

The Tabqa Dam -- also known as the Euphrates Dam, and Al-Thawra Dam (Dam of the Revolution) -- is as important for Syria as the massive Aswan Dam is for Egypt.

Like the latter, it was built with help from the former Soviet Union, a longtime ally of the Syrian regime.

Building began in 1968, and it was inaugurated in July 1973 during the reign of president Hafez al-Assad, father of the current leader Bashar al-Assad.

The Euphrates is the main source of water for agriculture and livestock in the region, and the dam has given Raqa an important role in the Syrian economy.

It was designed to generate 880 megawatts of electricity and provide irrigation for more than 600,000 hectares (1.5 million acres) of land. 

But high salt levels in the surrounding land have reduced the amount actually irrigated to less than a third. 

Source: AFP