Rescuers in helicopters tossed ropes and emergency aid to stranded Ethiopians on Thursday after many scrambled onto rooftops and trees to escape floods that have killed 900 and marooned tens of thousands.

Rushing waters have devastated large areas throughout the Horn of Africa nation since early this month as emergency services struggle to cope with overwhelming numbers of evacuees.

After two rivers burst their banks killing 870 people, Ethiopia has warned that more rivers have overflowed in the north, south, east and west, and the country's major dams were close to breaking point.

Medical relief agency Medecins Sans Frontieres (MSF) said flooding near the northern Lake Tana, the source of the Blue Nile River, had displaced 10,000 people, but the number could increase to 35,000 in coming weeks.

"About 2,000-3,000 people are being evacuated per day (from areas around Lake Tana)," Manuel Lopez Iglesias, head of mission for the Greek branch of MSF, told Reuters.

"We are expecting this number to grow in the coming days as these villages are evacuated."

Lopez said the situation was not likely to get better as the rainy season would last until September.

Rescuers said on Thursday they had saved some 6,000 people in the remote south where the Omo River burst its banks on Sunday killing at least 364 people.

"Rescue operations are still continuing, and people who have been marooned by the floods are being evacuated to safe areas," said Simon Macahele, director general of the state-run Disaster Prevention and Preparedness Commission (DPPC).

"Some six thousand people have been rescued from the (Omo) flood area."

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Helicopters, dispatched to the south, used ropes to save thousands of mainly local herders cut off by the waters and dropped emergency aid by air.

"An Ethiopian airforce helicopter detachment has begun airdropping food packets and shelter materials to people stranded by the floods," said Chief Inspector Tsegaye Mulunehe of the Omo regional administration.

Officials and aid workers fear the number of dead and homeless could be much higher due to the inaccessibility of the regions worst affected.

Africa's second most populous nation, Ethiopia has appealed for international aid to help overwhelmed emergency services.

After suffering from a drought last year, heavy rains have been battering the country in recent weeks.

Floods typically happen in lowland areas after rains in the June-August rainy season drench the highlands.

But the situation has been exacerbated by land cultivation, deforestation and overgrazing, aid workers say.

In the east, the Dechatu river burst its banks last week, killing 254 and leaving 250 missing, feared dead in Dire Dawa, 525 km (324 miles) east of the capital Addis Ababa.

As well as the Dechatu and Omo rivers, officials said the Awash River, some 300 km (190 miles) east of Addis, had burst its banks, leading to the evacuation of 7,000 people.

The Awash valley is home to three major sugar estates.

(Additional reporting by Jack Kimball in Nairobi)

Ethiopia began releasing water from dams taxed by two weeks of heavy rain to prevent them from bursting as the death toll from devastating floods climbed, state media said Tuesday.

With forecasters predicting yet more rain in this impoverished nation, floodgates from three major dams were opened, the Ethiopian News Agency reported.

Officials said they were releasing water from the dams in the central, southern and eastern parts of the country as a precautionary measure to prevent them from overflowing or bursting and causing uncontrolled flooding, according to the report.

Flooding caused by the heavy rains has left 626 people dead and displaced about 50,000, according to officials.

Meanwhile, officials issued fresh flood alerts in southern, northern and western Ethiopia as more bodies were recovered in the south, where the Omo River burst its banks on Aug. 13.

"This is a dangerous time for everyone and we are doing our best to prevent further deaths," Sisay Tadesse, spokesman for the government-run Disaster Preparedness and Prevention Agency told The Associated Press.

He said 1,300 people have been rescued by boat or helicopter in the south but he did not know the number of new bodies that had been recovered by rescuers.

Meanwhile, U.S. Navy personnel began relief operations in Dire Dawa, 310 miles east of Addis Ababa, where the first flash floods ripped through the town on Aug. 6, killing 256 and leaving 300 missing and feared dead.

U.S. naval engineers, who are part of the anti-terror Combined Joint Task Force Horn of Africa based in neighboring Djibouti, set up tents to shelter thousands of displaced residents. Sanitation facilities have also been built.

Waterborne diseases have compounded the rescue efforts, with 150 people having died countrywide from acute diarrhea and nearly 12,000 infected, the U.N.