Prime Minister Najib Abdul Razak is not one of those suspected of wrongdoing in the Swiss probe into misappropriation of US$4 billion from 1MDB, the Attorney-General's Office (OAG) of Switzerland said.

"In the ongoing criminal proceeding of the OAG, Mr Najib Razak is not one of the public officials under accusation," Andre Marty, spokesman for the OAG told the Nikkei Asian Review .

This comes after the Swiss OAG revealed it suspected former Malaysian officials and 'persons unknown' are involved in the misappropriation of funds from state-owned firms.

Marty told Nikkei that the phrase 'persons unknown' in Swiss law meant there was reasonable suspicion that an offence was committed but those who had committed it had yet to be identified.

This also comes as Singapore authorities said it has seized bank accounts in its probe on possible money laundering linked to 1MDB.

Singapore is also cooperating with Swiss investigators.

Meanwhile, in a statement late Monday, 1MDB said it has not been contacted by any foreign authorities on investigations into the firm's dealings.

"1MDB remains committed to fully cooperating with any lawful authority and investigation, subject to advice from the relevant domestic lawful authorities, and in accordance with international protocols governing such matters," it said.

Najib was last week cleared by Malaysian attorney-general Mohamed Apandi Ali of any wrongdoing related to the RM2.6 billion (US$681 million) donation and RM42 million channelled from state-owned SRC International into his personal bank accounts.

The donation was from the 'Saudi royal family' and of this, US$620 million was returned by Najib to the donor.

Deputy Minister Aziz Kaprawi said the US$61 million was used for the 13 th general election.

Najib has maintained he has never taken public money for personal use.

Apandi said Malaysia will assist the Swiss probe, although Deputy Prime Minister

Ahmad Zahid Hamidi is unhappy the Swiss OAG for releasing its media statement.