(Corrects first paragraph to reflect stated aim of engineering work)
By Rodi Said
TABQA DAM, Syria, March 29 (Reuters) - Islamic State shelled positions held by the U.S.-backed Syrian Democratic Forces (SDF) at the Tabqa Dam on the Euphrates River on Wednesday, forcing engineers to temporarily halt efforts to to ease the water pressure, a Reuters witness said.
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 engineers, who are working to open the spillways to relieve water pressure in the dam, later returned to work.
The SDF, an alliance of militias including the Kurdish YPG and Arab fighters, last week captured the northern part of the dam, which is a major strategic objective of the U.S.-backed campaign to isolate and capture the Islamic State-held city of Raqqa, some 40 km (25 miles), to the east.
The engineers were also at the dam on Tuesday carrying out an assessment as they try to open the two spillways, one of which is half open and the other is completely shut.
The SDF and U.S.-led coalition have said the dam is not in danger after the Syrian government on Sunday said it had been damaged by U.S. air strikes and could collapse, with the risk of catastrophic flooding.
Islamic State has also said the dam's operating systems were not working properly and it was vulnerable to collapse.
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. (Writing by Tom Perry; Editing by Louise Ireland)