Twelve bodies were recovered from the worst-hit Serbian town of Obrenovac, 30 km (18 miles) southwest of the capital, Belgrade.
"The situation is catastrophic," Prime Minister Aleksandar Vucic said, and warned that the death toll would likely rise.
Russian cargo planes carrying boats, generators and food joined rescue teams from around Europe and thousands of local volunteers in evacuating people and building flood defences after the River Sava, swollen by days of torrential rain, burst its banks.
Rains eased and flood waters receded on Sunday in some of the worst-hit areas of Serbia and Bosnia, but the River Sava was forecast to continue rising.
Serbia's EPS power utility said a fresh flood wave from the Sava and Mlava rivers threatened the Nikola Tesla and Kostolac power plants. Capacity has already been cut back at the Nikola Tesla plant in Obrenovac, Serbia's largest.
The Mlava overwhelmed sandbag flood defences on Sunday morning near Kostolac, threatening to flood coal mines and the plant itself. Power plant workers joined soldiers and police in trying to divert the water, digging up a road in one area.
Villagers nearby were evacuated, many of them refugees from the 1990s wars in Bosnia and Croatia. Kostolac currently covers 20 percent of Serbia's electricity needs.
Flooding had already cut Serbian power generation by 40 percent, forcing the cash-strapped country to boost imports.
Vucic said a fire and flooding of surface mines at the 1,300 megawatt (MW) Kolubara coal-fired power plant southwest of Belgrade had caused damage of "at least 100 million euros ($137 million)".
The economic impact of the floods is likely to be huge, devastating the agricultural sector vital to both the Serbian and Bosnian economies.
"These are the kind of waters not seen in 1,000 years, let alone 100," Vucic told a televised cabinet session.
Vucic said 12 bodies had been recovered from Obrenovac after waters dropped from a peak of some three metres (10 feet). At least three more have been reported dead elsewhere in Serbia.
In Bosnia, 19 people were confirmed dead by Saturday, with nine bodies recovered from the northeastern town of Doboj after what the regional police chief described as a "tsunami" of water 3-4 metres (10-13 feet) high.
A Reuters cameraman at the scene said half the town was still submerged. Bosnian soldiers distributed food and medical supplies by truck, boat and bulldozer. Cranes lifted medical workers into the top floors of some homes and removed stranded residents from others.
Zeljka Cvijanovic, prime minister of Bosnia's autonomous Serb Republic, compared the devastation to Bosnia's 1992-95 war, in which 100,000 people died.
"The damage is such that we cannot recall even after the war," she told reporters.
In Croatia, the government said one person had died and two were missing in flooded Sava river villages in an eastern corner of the country near Bosnia and Serbia. The army used amphibious vehicles to help evacuate some 3,000 people.
"I carried my kids out on my back, then waited 12 hours to be rescued myself," said 40-year-old Obrenovac resident Dragan Todorovic, who spent the night in a Belgrade sports hall with dozens of other families. "The house was new, built two years ago for 100,000 euros. What now"