Water rights have long been a source of friction in ties between the two countries, which share a more than 900-kilometre (560-mile) border.

Esmaeil Baqaei, spokesman for Tehran's foreign ministry, voiced on Friday "strong protest and concern over the disproportionate restriction of water entering Iran" due to the Pashdan Dam project.

He said in a statement that the Iranian concerns had been communicated "in contact with relevant Afghan authorities".

"Exploitation of water resources and basins cannot be carried out without respecting Iran's rights in accordance with bilateral treaties or applicable customary principles and rules, as well as the important principle of good neighbourliness and environmental considerations," Baqaei added.

Abdul Ghani Baradar, Afghanistan's deputy prime minister for economic affairs, said in a video statement last month that the Pashdan project was "nearing completion and water storage has commenced".

According to the video, the dam in Herat province will store approximately 54 million cubic meters of water, irrigate 13,000 hectares of agricultural land and generate two megawatts of electricity.

In April, Baradar said the dam was a "vital and strategic project" for Herat province.

The foreign ministry statement on Friday follows remarks by an Iranian water official, similarly criticising the dam construction.

"The situation has led to social and environmental issues, particularly affecting the drinking water supply for the holy city of Mashhad", Iran's second-largest and home to a revered Shiite Muslim shrine near the Afghan border, national water industry spokesman Issa Bozorgzadeh was quoted as saying on Monday by official news agency IRNA.

Harirud River, also known as Hari and Tejen, flows from the mountains of central Afghanistan to Turkmenistan, passing along Iran's borders with both countries.

In his statement, Baqaei said Iran expects "Afghanistan... to cooperate in continuing the flow of water from border rivers" and to "remove the obstacles created" along their path.

In May 2023, Iran issued a stern warning to Afghan officials over another dam project, on the Helmand River, saying that it violates the water rights of residents of Sistan-Baluchistan, a drought-hit province in southeastern Iran.