"The preliminary reason for the collapse was too much snow on the roof," the agency said, adding that the radiation situation is "within the norm" and nobody was harmed in Tuesday's incident.
The roof was constructed after the 1986 disaster but is not part of the sarcophagus structure covering the reactor, it said.
However the collapse underlines concerns about the condition of the now defunct nuclear plant over two-and-a-half-decades after the world's worst nuclear disaster.
Part of the roof and some of the walls at the plant's machine room, close to the sarcophagus that seals the reactor number four which melted down in the 1986 accident, fell under the weight of the snow.
The area of the accident is estimated about 600 square metres, (6,500 square feet), the emergency agency said.
A statement on the website of the power station described the accident as the "partial failure of the wall slabs and light roof of the Unit 4 Turbine Hall."
It said that the damaged structure was not critical part of the protection structures at the power plant.
Chernobyl is only around 100 kilometres (60 miles) from Ukraine's capital Kiev and lies close to the borders with Russia and Belarus. The area around the plant is still very contaminated and is designated as a depopulated "exclusion zone".
Amid concerns about the state of the sarcophagus, an arch-shaped structure called the New Safe Confinement is being built nearby to slide over the existing sarcophagus covering the reactor.
The EBRD is administering the fund to build the shelter with the help of donor contributions. When it is finished in 2015, the structure will weigh 20,000 tonnes and span 257 metres (almost 850 feet).
Two workers were killed by the April 26, 1986 explosion and 28 other rescuers and staff died of radiation exposure in the next months. Tens of thousands of people needed to be evacuated and fears remain over the scale of damage to people's health.
In 1986 and 1987, the Soviet government sent more than half a million rescue workers, known as liquidators, to clear up the power station and decontaminate the surrounding area.
However the total death toll from Chernobyl remains a subject of bitter scientific controversy, with estimates ranging from no more than a few dozen deaths directly attributable to the disaster to tens of thousands.
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