(VOA) -- U.S.-backed forces in Syria are closing in on a strategic dam on the Euphrates River occupied by Islamic State fighters as part of their offensive to free Raqqa, IS's de facto capital.
Commanders of the Syrian Democratic Forces (SDF), a Kurdish-led alliance, say their objective in the coming days is to recapture the Tabqa dam, the largest in Syria, which is nearly 45 kilometers upstream from Raqqa.
The Pentagon said that SDF forces have made significant progress in western Raqqa and that they are only seven kilometers away from the dam, also known as the Baath Dam.
"Liberating the dam will allow us to control the town of Tabqa as well," said Nasir Hajj Mansur, an SDF commander who is involved in the ongoing offensive. "It will also break the back of (IS) as it is one of the last defense lines before Raqqa city."
SDF fighters Friday took control of the nearby strategic Jaabar Castle. The historic citadel was used as a "training and planning center" by IS, an SDF spokesperson said.
The dam, which IS has controlled since 2014, is a hydropower facility that is one of the main sources of electricity in the country. It has also been vital for the irrigation of farms in the fertile region around Raqqa.
Tabqa town, located three kilometers away from the dam, is considered a major security hub for IS fighters. It has two large prisons where several Kurdish commanders and female prisoners enslaved by IS have reportedly been held for months.
In anticipation of an imminent assault, IS has reportedly moved nearly 150 prisoners from the prison to a different location, the Britain-based Syrian Observatory for Human Rights reported Friday.
If the Kurdish-led forces succeeded in retaking the Tabqa, it would be the second dam on the Euphrates to be captured from IS fighters. With the help of U.S. airstrikes, SDF fighters took control of the Tishrin Dam in December 2015.
IS is expected to create as many hurdles as possible for the U.S.-backed forces heading to the dam.
"Similar to what they did with Tishrin Dam, IS will likely destroy all roads and supply lines that lead to the Tabqa Dam to delay the advancement of SDF fighters," said Baz Ali Begari, a journalist at Syria's Qasioun news agency, who closely reports on the Raqqa offensive.
There are fears that if IS decides to sabotage the 60-meter-high dam, it could cause a humanitarian crisis in eastern Syria.
"The dam was built nearly 50 years ago and it hasn't been maintained for a long time," said Tammam Baroudi, an agricultural economist at the Syrian Economic Forum.
Baroudi fears that heavy fighting between IS and opposing forces could damage the dam severely.
Military commanders on the ground acknowledge the risks involved in retaking the dam from IS.
"Blowing up the dam (by IS) could put the entire region, including Deir Ezzor, in danger," said Kurdish commander Mansur. "That's why we and our partners in the U.S.-led coalition are very careful not to target the dam directly."
Recent U.S. airstrikes have struck IS targets around the dam, including IS fighting positions and weapons depots in Tabqa.