Fiji's government has offered its condolences to the families of those who died in an overnight bus crash on the main island of Viti Levu.

Ten people were killed in the accident and dozens of others injured, a number of them critically.

It's been confirmed that a pregnant woman and her two young children were among the dead.

The bus was travelling from the capital Suva to Nadi when it crashed and burst into flames.

An official investigation into accident is under way.

But Fiji police spokesman, Atunaisa Sokomuri, has told Radio Australia's Pacific Beat it's not yet clear what caused the smash.

"The accident happened near midnight, and there are a lot of conflicting stories at present with a lot passengers giving in their statements also the driver being interrogated by police," he said.

"But we'll wait and we'll see how all the statements add up before we really confirm what really happened on that fateful night."

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