Syrian engineers are working to open spillways and ease pressure on a major dam across the Euphrates River, amid a pause in a US-backed assault to capture it from Islamic State.
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 US-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 US 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 US-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.
US-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.