TABQA DAM, Syria (Reuters) - Syrian engineers worked on Tuesday to open spillways and ease pressure on a major dam across the Euphrates River during a pause in a U.S.-backed assault to capture it from Islamic State (IS) militants, a Reuters witness said.

Syrian Democratic Forces (SDF) fighters stand at the northern part of the Tabqa Dam on the Euphrates River, Syria March 28, 2017. REUTERS/Rodi Said

The Tabqa Dam is a key strategic target in the military campaign to isolate and capture the eastern Syrian city of Raqqa, Islamic State’s biggest urban stronghold.

Engineers arrived from the dam’s northern entrance, which the U.S.-backed Syrian Democratic Forces (SDF) alliance captured last week. The dam’s southern reaches remain in IS hands.

U.S.-led coalition aircraft could be heard overhead as SDF forces manned positions on the dam. Coalition forces in armored vehicles were also seen in the area.

Inspections at the dam were being carried out 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 said the dam’s operating systems were not working properly and it was vulnerable to collapse. The coalition later said it saw no imminent danger to the dam, unless the militants planned to blow it up.

No fighting could be seen or heard at the dam on Tuesday, according to the Reuters photographer, who was at the site for about 90 minutes.

Islamic State said in a statement on Tuesday that the maintenance team responsible for the dam had been killed in a U.S.-led air strike on Monday, the day the SDF paused its assault.

The coalition press office did not immediately respond to a Reuters request for comment. The main operations buildings for the dam along with its technicians are located on the southern bank controlled by Islamic State.

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The UK-based Syrian Observatory for Human Rights said it had learned from trusted sources that the engineer responsible for administering the dam had been killed along with another technician, but did not detail how.

The Observatory on Tuesday cited sources saying Islamic State had sent 900 fighters from Raqqa to confront the SDF as it advances on the city on several fronts. It was not clear where they had been sent to.

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.

RAQQA OFFENSIVE

U.S.-backed forces are also battling Islamic State for control of the Iraqi city of Mosul. Defeats in Mosul and Raqqa would deal a heavy blow to Islamic State in the cities from where it declared its “caliphate” across Syria and Iraq in 2014.

The SDF seized Tabqa air base on Sunday, the first such facility to fall under the control of Syrian Kurdish militias and their allies that now control swathes of northern Syria after six years of multi-sided civil war.

In comments to the London-based Arab newspaper al-Hayat published on Tuesday, the head of the Kurdish YPG militia said 16,000-17,000 Arab and Kurdish fighters would take part in the assault on Raqqa.

YPG commander Sipan Hemo also said U.S. Apache attack helicopters “will participate in providing air support to our forces”.

Hemo told Reuters earlier this month that the operation to storm Raqqa would start in early April and last no more than a number of weeks. Echoing that assessment, he told al-Hayat “we will liberate Raqqa in weeks or one month, not more”.

Turkey is fiercely opposed to the YPG’s role in the Raqqa offensive, and has been lobbying the United States to drop the Kurds and instead work with Ankara and its Syrian rebel allies to take the city. Ankara sees the YPG as an extension of a Kurdish militant group that is fighting an insurgency in Turkey.

Washington says a final decision on when and how Raqqa will be taken has yet to made.