Egypt “categorically rejects” the commencement of the second filling of the Grand Ethiopian Renaissance Dam’s (GERD) reservoir, of which Ethiopia notified Egypt earlier on Monday.

The step by Ethiopia could escalate tensions with downstream countries Egypt and Sudan, who have raised the GERD issue with the United Nations Security Council (UNSC), which is set to convene to discuss the issue this week.

The commencement of the second filling, which was announced via a letter sent by Ethiopian Irrigation Minister Sileshi Bekele to his Egyptian counterpart Mohamed Abdel-Ati on Monday, comes despite the lack of a binding agreement with Egypt and Sudan.

In a response letter to Bekele, Minister Abdel-Ati asserted Egypt's “categorical” rejection of Ethiopia’s unilateral move, which he described as a "blatant and dangerous" violation of international laws and norms as well as the Declaration of Principles agreement signed between the three African countries in 2015.

"The unilateral measure is an explicit and serious breach of the Declaration of Principles Agreement, as well as a violation of international laws and norms that govern projects built on the common basins of international rivers, including the Nile River," Abdel-Ati added.

The utilisation of the Nile River’s resources is regulated by agreements that obligate Ethiopia to respect Egypt's water rights and interests and not to harm to them, Abdel-Ati said in a statement by the Egyptian Ministry of Irrigation.

The Egyptian Ministry of Foreign Affairs has forwarded a copy of the Ethiopian letter to the president of the UNSC.

"This serious development reveals once more Ethiopia’s ill intention and its insistence on taking unilateral measures to force a fait accompli [by] filling and operating the Renaissance Dam without an agreement that takes into account the interests of the three countries and limits the damages of this dam on the two downstream countries," the foreign ministry said.

The ministry said it expects the Ethiopian measure will "increase the state deadlock and tension in the region, and will create a threatening situation to security and peace at the regional and international levels."

Addis Ababa aims to store up to 13.5 billion cubic metres of water in the GERD’s reservoir during the current year's flood season, which started in July.

The UNSC is set to meet on Thursday upon the request of Egypt and Sudan, who raised the matter with the 15-member body after the African Union-sponsored tripartite negotiations with Ethiopia reached a deadlock in April.

Egypt and Sudan have been negotiating with Ethiopia for a decade to reach a legally binding agreement on the filling and operation rules, with both downstream countries blaming Addis Ababa for the failure of the talks.

Egypt, which depends on the Nile for over 95 percent of its freshwater, fears the dam will significantly affect its water share if a legally binding deal is not reached.

Sudan fears the unilateral filling of the reservoir will threaten the lives of millions of its people living downstream of the dam, jeopardise the operational safety of its own dams, and consequently risk Sudan's national security.

Ethiopia, which hopes the controversial multi-billion-dollar hydropower dam will support its economic development goals, seeks to sign non-binding guidelines on the dam's filling and operation rules, as opposed to both downstream countries, who seek a binding deal.