Ethiopia has begun diverting the Blue Nile as part of a giant dam project, officials said on Wednesday, prompting talks in Cairo between downstream states Sudan and Egypt.
The $4.2 billion (3.2 billion euro) Grand Renaissance Dam hydroelectric project had to divert a short section of the river -- one of two major tributaries of the Nile -- to allow the main dam wall to be built.
"To build the dam, the natural course must be dry," said Addis Tadele, spokesman for the Ethiopian Electric Power Corporation (EEPCo), a day after a formal ceremony at the construction site.
The river is being shifted about 550 metres (yards) from its natural course, Addis said, but stressed that water levels would not be affected.
"There is no problem with the river levels," he said.
The first phase of construction is expected to be complete in three years, with a capacity of 700 megawatts.
Once complete, the dam will have a capacity of 6,000 megawatts.
Both Sudan and Egypt, arid nations that rely heavily on the Nile for water, particularly for agriculture, are extremely sensitive about projects that could alter the flow.
Sudan's minister of irrigation, Osama Abdallah al-Hassan, arrived in Cairo for talks on the Ethiopian move, Egypt's official MENA news agency said.
The Egyptian cabinet also met and issued a statement saying it opposed all projects which could affect the flow of the Nile.
It said it had planned "several scenarios" depending on the outcome of an assessment to be conducted by the three governments.
In Khartoum, the foreign ministry said Sudan would not be affected by the project, stressing in a statement that there are agreements and consultations between Sudan, Egypt and Ethiopia.
"Sudan respects the agreements to cooperate with those two countries (Egypt and Ethiopia) in matters that concern sharing the waters of the Nile and sharing mutual revenues," the ministry said.
Egypt believes its "historic rights" to the Nile are guaranteed by two treaties from 1929 and 1959 which allow it 87 percent of the Nile's flow and give it veto power over upstream projects.
But a new deal was signed in 2010 by other Nile Basin countries, including Ethiopia, allowing them to work on river projects without Cairo's prior agreement.
EEPCo insists the project will not impact downstream needs, claiming the dam will provide "highly regulated outflows" by reducing floods at peak times and providing more water during otherwise low flows.
The dam project, in Ethiopia's northwestern Benishangul-Gumuz region near the border with Sudan, was launched in April 2011 by late prime minister Meles Zenawi.
Funding is being raised publicly, with the state raising funds locally, and no external financing has been provided.
Ethiopia is constructing a series of dams to produce hydroelectric power for both domestic consumption and export.
EEPCo has plans to establish transmission lines to neighbouring countries, including Sudan, Kenya and Djibouti.
One of Ethiopia's deputy prime ministers, Demeke Mekonnen, officially launched the river diversion on Tuesday, alongside EEPCo chief Mihret Dibebe.
When completed, the dam wall will stretch almost 1.8 kilometres (about one mile) in length and 145 metres (475 feet) in height.