Reinsurance giant Swiss Re on Tuesday said that Thailand's worst flooding in decades would cost it around $600 million, but warned that the estimate could be revised.

"Water levels remain high in certain regions, making it difficult to correctly estimate the losses," the company said in a statement.

"The impact of the floods on the Thai economy and on businesses active there is likely to be big and could last for a certain amount of time," said Brian Gray, chief underwriting officer at the Swiss company.

The floods had forced several important industrial complexes to close, with factories under several metres of water for weeks and unable to produce key components for automakers and electronics manufacturers, he said.

Some 1,500 industrial sites were affected by the floods, the firm added.

Three months of unusually heavy monsoon rains have inundated large swathes of Thailand, killing at least 675 people and affecting the homes and livelihoods of millions.