The small town of Hasankeyf in the southeastern Batman province, home to 3,000 residents, will disappear as the lake is filled for the Ilisu project.

While some residents welcome the development as a boost to the local economy, many are concerned over the loss of heritage.

"They have closed the dam and the water is rising," said Ridvan Ayhan, spokesman for the "Keep Hasankeyf Alive" collective, an activist group which opposes the dam.

The dam has been built downstream of the Tigris river, causing concern in Iraq, which shares the river, that it will add to the region's water shortages.

But the dam is a central part of Turkey's long-running Southeastern Anatolia Project (GAP), which aims to develop one of the country's poorest regions through energy and irrigation.

Ayhan said he had satellite images showing the water covered a 20-kilometre area in a process the group believed began between July 17 and 19.

"We are asking the authorities to empty the dam, let the water flow. They haven't made any statement," Ayhan said, adding the situation was "extremely worrying".

He said he was unsure how long the process to submerge the town would take, but that it would probably be several months.

The DSI organisation responsible for Turkey's dams did not immediately respond to a request for comment.

The government argues that Hasankeyf's monuments have been saved, including the 1,600-tonne Artuklu Hamam bath house and remnants of a 14th-century Ayyubid mosque, by relocating them to high ground.

The European Court of Human Rights (ECHR) in February dismissed an appeal by activists to stop the project, saying there was no universal right to protect cultural heritage.