Warplanes targeted maintenance workers of the Euphrates Dam in Syria's Raqqa province, a monitor group has said.

Airstrikes believed to be conducted by the US-led anti-terror coalition killed Engineer Ahmad Husain and another maintenance worker in Tabqa city on Tuesday, Xinhua news agency quoted Syrian Observatory for Human Rights as saying.

Other workers were wounded, and their condition was still unknown, the London-based watchdog group said.

The Euphrates Dam has been rendered out of service as a result of the US-led airstrikes on the facility, amid reports that maintenance workers were supposed to assess the damage to fix it.

The dam controls 13 billion square metres of water in the Assad Lake, which derives its water from the Euphrates river.

The dam is 60 m high and 4.5 km long and is the largest dam in Syria. Its construction led to the creation of Lake Assad, Syria's largest water reservoir.

It was constructed between 1968 and 1973 with the help from then the Soviet Union.