Former premier Dr Mahathir Mohamad has raised questions on why Prime Minister Najib Abdul Razak's receipt of RM2.6 billion in "donations" have not raised red flags with the banks, locally and abroad.

In a posting on his blog today, Mahathir thus far, apart from the freezing of two bank accounts in Singapore, nothing has been heard of since.

"Is that all? Surely the holder of the account should be investigated. But so far, nothing is heard. Do the banks get to keep the money?

"If it is not reported to the authorities for action to be taken, then effectively the money has been laundered," he said.

Najib took more than a month before admitting to Wall Street Journal's claim that he had received nearly US$700 million in his private bank accounts with AmBank.

The funds were received in 2013, before the general election. Najib had suggested that the funds were used as political funding.

Najib has not disclosed how much funds were used, but what is publicly known is that the bank accounts were closed in 2014.

Whistleblower website Sarawak Report claimed that on the same day that the accounts were closed, the balance of US$650 million were transferred to a bank in Singapore.

This claim has not been denied by Putrajaya.

Mahathir argued that it was the responsibility of banks the world over to scrutinise large amounts of money deposited into their accounts.