Floodwaters crept lower in the shellshocked Balkans Thursday to reveal widespread devastation after the region's deadliest natural disaster since records began more than a century ago.

As thousands of relief workers began an immense clean-up operation, the first of some 150,000 people evacuated over the past week were allowed to return to their towns and villages to pick up the pieces.

But many places as well as vast tracts of farmland remain under muddy brown water, large areas are without power, and those returning -- if their home still exists -- returned to a disaster area.

Fifty-one people are confirmed to have died, but this could rise.

There is a risk of epidemics from rotting livestock carcasses, while dislodged landmines from the 1990s Yugoslav wars in Bosnia may be lurking in the mud and debris.

Late Tuesday one such device exploded in northern Bosnia, although no one was injured. There are estimated to be more than 120,000 unexploded mines littered around Bosnia.

With temperatures approaching 30 degrees Celsius (86 Fahrenheit), health officials were working hard to recover dead livestock. They were also spraying to try to prevent a plague of mosquitoes.

The deluge began last week when record amounts of rain lashed south-eastern Europe, turning the Sava river and its tributaries in Bosnia, Serbia and Croatia into raging torrents that burst their banks.

In scenes reminiscent of the chaos of the 1990s conflicts, more than 1.6 million people across the Balkans were affected.

In Serbia more than 30,000 people were evacuated, with some 1,750 buildings destroyed and 2,250 flooded -- not counting the town of Obrenovac, the worst affected.

- 'Worse than the 1990s war' -

In Bosnia 100,000 had to flee, while in Croatia, which escaped the worst damage, authorities said 38,000 people have been affected, with some 2,000 houses and 199 farms destroyed.

In the northern Bosnian town Samac, still almost fully covered with muddy water, Ruzica Aleksic was still awaiting rescuers on a balcony of her first-floor flat.

"This is worse than during the 1990s war, then at least we could escape," said Aleksic.

Only 700 of its 26,000 inhabitants have still remained in the town, said Colonel Mirsad Adzic, rescue service commander. Two people died in the town and two others are missing.

In Obrenovac, one of the hardest hit towns in Serbia with half of its population evacuated, inhabitants began returning to assess the damages in their homes on Thursday.

Its main streets have already were cleared of debris and mud. A few people were cleaning their shops, throwing away shelves or goods ruined by the water, AFP photographer said.

- Hundreds of millions of euros -

Aside from the human tragedy, the Balkans' worst floods since records began more than a century ago are also set to deal a heavy blow to the region's economy.

"It seems clear to me that the damages are going to run into hundreds of millions of euros (dollars)," Serbia's Prime Minister Aleksandar Vucic told a meeting in Belgrade of international donors including the World Bank.

Thirty-nine Serbian towns and villages have been hit, 80 bridges have been destroyed and another 200 damaged, while 3,500 kilometres (2,175 miles) of roads will need repairs, he said.

In Bosnia, Ahmet Egrlic, head of the chamber of commerce, said Thursday that the floods could slash economic output by 30 percent and that hundreds of jobs were at risk.

"Exports are definitely going to fall because of this catastrophe. Revenues are going to plunge, which will hit tax receipts," Egrlic said.

Bosnia will "need international aid," Igor Radojicic, parliamentary speaker in the Bosnian Serb entity Republika Srpska, told AFP.

"Lots of people are not going to have a home any more," he said.