"We have started sending out help, emergency items," Sisay Tadesse, spokesperson for the government’s Disaster Prevention and Preparedness Agency (DPPA), said.
He added that nearly 20,000 tonnes of relief aid was being sent by truck on Monday.
"Sixty-seven dead people have been counted at the moment and we estimate the number of people who live in the affected zones at 280,000," a humanitarian worker said from the town of Gode, about 650 kilometres southeast of the capital, Addis Ababa.
Nearly half of the population living near the banks of the Wabe Shebelle River in Ethiopia's southeastern Somali State had been forced from their homes, another aid worker said, adding that the number of people affected by the floods could still rise.
On Monday, the DPPA announced that 15 people were killed after the Wabe Shabelle burst its banks during a torrential rainfall that has pounded the area since 23 October.
By Wednesday, the water level had stabilised at 12 metres, twice its normal height, after the rains eased on Tuesday.
"The situation is very tenuous, more and more people are losing their nerve and asking when help will arrive because they believe the water will continue to rise," the humanitarian worker added. "Most of the affected people are inaccessible by road and we urgently need helicopters to access people and assess the situation."
The floods mainly hit the towns of Kelafo and Musthale, 80 kilometres and 150 kilometres south of Gode, which lie along the river.
At least 639 people were killed in August when unusually heavy rains triggered massive flooding in the eastern, northern and southern regions of the country. A total of 357,000 people were affected by those floods, which the United Nations humanitarian agencies said were the worst in decades.
Ethiopia, home to 77 million people, has faced heavy floods and droughts in recent years along with other countries in the Horn of Africa. Flooding usually occurs in the lowlands after heavy rains in the June-September season drench the highlands. This year's flooding was especially damaging because it followed a severe drought.
eo/oss/jm
This article was produced by IRIN News while it was part of the United Nations Office for the Coordination of Humanitarian Affairs. Please send queries on copyright or liability to the UN. For more information: https://shop.un.org/rights-permissions