TABQA DAM, Syria (Reuters) - Spillways at the Tabqa Dam in Syria are working normally after engineers managed to carry out repairs, a local alliance of Kurdish and Arab militias said on Wednesday, despite shelling by Islamic State that temporarily halted their work.

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A media official with the U.S.-backed Syrian Democratic Forces (SDF) militia bloc said repairs continued after the shelling incident, witnessed by Reuters, and the spillways were now functioning normally.

The dam is a major strategic objective of the SDF’s U.S.-backed campaign to isolate and capture the Islamic State-held city of Raqqa, some 40 km (25 miles) to the east.

The SDF began an assault to capture it last week after the coalition landed some of its fighters on the southern side of the Euphrates near Tabqa, leading to its capture of an air base on Sunday. On Wednesday they cut the road from Tabqa to Raqqa, the SDF said.

Islamic State and the Syrian government both said on Sunday that the hydroelectric dam on the Euphrates river was vulnerable to collapse after air strikes by the U.S.-led coalition fighting the militants in the country’s north.

The SDF and the coalition have both said the dam is not in danger, but they paused their assault to capture it on Monday and gave engineers access to spillways at the northern end of the 4-km-long (2.5 mile) dam in a bid to relieve water pressure.

Islamic State fired from the southern end of the dam, which it controls, and at least two explosions were heard. No one was injured. The south end covers the main channel and flood gates, as well as operational buildings and a hydroelectric plant.

The engineers were also at the dam on Tuesday carrying out an assessment of its soundness as they tried to open the two spillways, one of which was half open and the other completely shut.

The Syrian government has warned a collapse in the dam could cause catastrophic flooding in the cities and towns in the Euphrates valley downstream. Those areas represent Islamic State’s most important remaining possessions in Syria.

The head of the Kurdish YPG militia, fighting in the Raqqa campaign as part of the SDF alliance, has said the final assault on the city will begin in early April.