* Engineers visit Tabqa Dam, aim to open spillways
* Dam is a strategic target ahead of Raqqa offensive
* Observatory: IS sends 900 fighters to frontlines near Raqqa
* YPG head: 16,000-17,000 Arab and Kurdish fighters to take part
* Graphic: tmsnrt.rs/2l02TCA
By Rodi Said
TABQA DAM, Syria, March 28 (Reuters) - Syrian engineers worked on Tuesday to open spillways and ease pressure on a major dam across the Euphrates River, amid a pause in a U.S.-backed assault to capture it from Islamic State, a Reuters witness said.
The Tabqa dam is a key strategic target in the military campaign to isolate and capture the Syrian city of Raqqa, Islamic State's biggest urban stronghold.
The 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 the hands of the militants.
Coalition aircraft could be heard overhead as SDF fighters manned positions on the dam. Coalition forces in armoured vehicles were also seen in the area.
Work on the dam was 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 U.S.-led 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.
The UK-based Syrian Observatory for Human Rights 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.
U.S.-backed forces are also battling Islamic State for control of the Iraqi city of Mosul. Defeats in both would deal a double 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 16,000-17,00 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 Washington to abandon the Kurds and instead work with Ankara and its Syrian rebel allies to take the city. Turkey 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.
Writing by Tom Perry in Beirut; editing by Richard Lough