Bosnia's record floods in May have caused damage estimated at two billion euros ($2.7 billion), seriously affecting the country's economy, an international expert group said on Wednesday.

The cost is equivalent to some 15 percent of Bosnia's 2013 gross domestic product (GDP) and will cause the economy to contract by 1.1 percent this year, compared to the growth of 2.2 percent that had been predicted before the floods, they said.

The estimates were made by experts from the EU, United Nations and World Bank along with local authorities.

It is only an "initial overall assessment of recovery needs" and "more verification work is needed" over the coming fortnight to provide more detailed numbers by sector, said a spokesman of the EU Delegation to Bosnia, Andy McGuffie.

An initial estimate by the European Bank for Reconstruction and Development (EBRD) in late May had put the damage at around 1.3 billion euros ($1.8 billion).

The new figures will serve as a basis for an international donor conference expected in July in Brussels aimed at raising money for reconstruction in Bosnia and Serbia, which was also severely affected by the floods.

The EBRD estimated that damage in Serbia was between 1.5 and 2 billion euros ($2-2.7 billion)

The head of the expert group, Ricardo Zapata Marti, said it would have "an important impact on the macroeconomic performance of the country".

"This economy was not going very fast before the disaster, now it's going to decrease," he said.