UN Development Programme official Cihan Sultanoglu yesterday said immediate needs include shelter, food, water, health and agriculture supplies, medicine, debris removal and mine clearance.
She said 800 square kilometres (308 square miles) of flood-affected areas are suspected to contain mines and unexploded ordinance, making the clean-up operation costly, difficult and lengthy.
More than 75,000 homes are estimated to have been damaged and up to 2,000 destroyed, and 60,000 children affected, she said.
Sultanoglu singled out a need to prioritise the repair of education facilities before the school term resumes on September 1.
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"The UN's initial estimate of the costs to cover the immediate priority needs during the first six months is approximately USD 210 million, and again I am only referring to Bosnia Herzegovina," she said.
Mid- to long-term costs are expected to be much higher but have yet to be assessed fully, she said.
UNDP has already received USD 1.2 million for two projects on mine clearance and debris clearance, she said.
"The challenges ahead are many, but they are not insurmountable if we act collectively," she said.
Torrential rains hit Serbia, Bosnia and Croatia in mid-May, killing 77 people in the worst floods and landslides in more than a century.
Bosnian authorities have warned against the danger of landmines left over from the Bosnian war of the 1990s, which may have been dislodged by flood water and landslides.