A dam in eastern Kenya is close to bursting under the strain of waters swollen by torrential rains, United Nations staff have warned.
The floods have already forced thousands away from their homes
Unusually heavy seasonal rains have raised the water level in the River Tana, near the town of Garissa.
A spokeswoman for the UN said the dam was "on the brink of bursting", and floodgates had to be opened to stop it from buckling and breaking.
Heavy rain is forecast to continue for weeks in Kenya, Ethiopia and Somalia.
Between 1.5 million and 1.8 million people have already suffered under the heavy rains and severe flooding, the UN says.
The rains and flash flooding followed a long period of drought that left plains and farmland dried out and unable to absorb water.
Much of the rainfall in eastern Kenya has found its way into the Tana, raising water levels both upstream from the dam and downstream, in the portion of the river likely to be further bloated by any opening of the dam.
Thousands of people living downstream would be affected by opening the dam, but officials believe a failure to act could have worse consequences.
"That's a major worry now because if a gap is not opened in the dam on the Tana, it risks breaking," said Elisabeth Byrs, of the UN's humanitarian affairs office.
Ms Byrs said the condition of the dam was the "biggest concern" in Kenya at present, more serious than destroyed roads, cut-off villages, or the threat of waterborne diseases.
There are fears that stagnant water across the three countries could provide a breeding ground for cholera, malaria and dysentery.
"We have a serious situation, and the humanitarian situation is going to get worse," Ms Byrs told reporters.