Four East African states have signed an agreement to seek more water from the River Nile - a move strongly opposed by Egypt and Sudan.
Under colonial-era accords, the two countries get 90% of the river's water.
Upstream countries including Uganda, Rwanda, Tanzania and Ethiopia say it is unfair and want a new deal but nothing has been agreed in 13 years of talks.
A further three countries were represented at the meeting in Entebbe, Uganda, and may sign up later.
BBC East Africa correspondent Will Ross says there is a danger that the split could hamper any further efforts for all nine countries involved to negotiate how the waters should be shared.
The BBC's Wyre Davies in Cairo says that for Egypt, water is a matter of national security.
"If we don't have an agreed co-operative framework, there will be no peace," Kenya's director of water resources John Nyaro told the BBC before the meeting.
"Where there is no rule of law, the rule of the jungle does not provide peace."
Ethiopia, Tanzania, Uganda, and Rwanda signed the agreement in Entebbe, which would lead to experts determining how much water each country would be entitled to.
Kenya did not sign the agreement as its minister could not attend. Like Burundi and the Democratic Republic of Congo, it sent officials to Entebbe.
Ethiopia, for example - the source of the Blue Nile - contributes an estimated 85% of the river waters but is able to make relatively little use of its natural resource.
Rwanda's Environment Minister Stanislas Kamanzi told the BBC: "Egypt has been requesting to defer the signing of the Cooperative Framework Agreement - we couldn't wait any longer, since we have been negotiating for over 10 years."
Egypt and Sudan say they will not sign a new deal unless they are first guaranteed an exact share of the water.
Ahead of the meeting, Ahmed el-Mufti, the legal counsel for Sudan's delegation, told Reuters news agency that all nine countries were close to an agreement, so there was no need for the upstream countries to sign their own deal.
He also said Egypt and Sudan needed water more than those in more fertile regions.
"They have a lot of rain: This is nature," he said. "They do not need the water. Here in Sudan we need water."
Egyptian Foreign Minister Ahmed Abul Gheit has warned that water rights were a "red line" and threatened legal action if a separate deal is reached, the AFP news agency reports.
Egypt's farmers are almost wholly dependent on the River Nile and its water.
The BBC's Will Ross says that, with populations soaring, demand for water increasing and climate change having an impact, there are warnings that wrangling over the world's longest river could be a trigger for conflict.
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