KUALA LUMPUR (THE STAR/ASIA NEWS NETWORK) - Prime Minister Najib Razak is not a suspect in the US$4 billion (S$5.7 billion) probe into state investor 1Malaysia Development Berhad (1MDB), said the Office of the Attorney-General (OAG) in Switzerland.

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

He added that the phrase "persons unknown" in Swiss law refers to criminal proceedings where there is a reasonable suspicion that an offence has been committed, but the accused is not known.

The OAG opened criminal proceedings last August against two former 1MDB officials as well as "persons unknown".

Last week, Switzerland's chief prosecutor, Attorney-General Michael Lauber, said he had formally asked Malaysia for help with his probe into possible violations of Swiss law by 1MDB, saying suspected misappropriations amounted to about US$4 billion.

Mr Lauber said the request pertained to possible violations of Swiss laws related to bribery of foreign officials, misconduct in public office, money laundering and criminal mismanagement at 1MDB.

His statement came after Malaysian Attorney-General Mohamed Apandi Ali absolved Mr Najib of any criminal wrongdoing in the probe in SRC International, a former subsidiary of 1MDB, and RM2.6 billion (S$888 million) political donation.

Mr Apandi later responded to Mr Lauber, saying his office would take all possible steps to follow up and collaborate with Switzerland and "we look forward to their investigations and materials through the normal channels".