Bangkok: Mystery has deepened over the circumstances in which almost $US700 million ($990 million) was secretly deposited in the personal bank account of Malaysian Prime Minister Najib Razak.

The country's attorney-general, Mohamad Apanda Ali, has declared the money was a donation from one of the nine sons of the late Saudi king Abdullah, but insists the motive is nobody else's business.

"Why are people asking for the reason for the donation? You have to ask the donor. He has billions in his coffers … if he wanted to give the money, what's the problem?" Mr Apandi told the Sin Chew Daily newspaper.

Saudi Arabian Foreign Minister Adel al-Jubeir says he does not think the money came from the Saudi government or that it was a political donation.

"It is a private Saudi citizen, I believe, and the funds went into an investment in Malaysia," Mr Jubeir told The New York Times.

The Times also quoted one member of the Saudi royal family and one associate of the family as saying the money had come from a Saudi prince and confirmed it was not a donation.

The associate questioned the reported sum but said it was part of a business deal.

For months Mr Najib has ignored the demands of opposition MPs to explain how he came by the money, which if it was a political donation would be one of the largest ever known.

"The matter has been put to rest," Mr Najib said late in January, after Mr Apandi cleared him of any wrongdoing. "It is time for us to reunite and move on."

The Wall Street Journal has reported the money flowed to Mr Najib's accounts through an anonymous British Virgin Islands company and a Swiss private bank wholly owned by an Abu Dhabi sovereign wealth fund that is intertwined with Malaysia's heavily indebted sovereign fund 1Malaysia Development Berhad (1MDB), which Mr Najib established in 2009 and still oversees through chairmanship of an advisory committee.

Mr Apandi has claimed that Mr Najib returned  $US620 million of the "donation" five months after it was sent in 2013, but refuses to provide any further details, fuelling speculation in Malaysia, where Mr Najib remains in office with the support of powerful division chiefs of his ruling party who have reaped the benefits of money politics over decades.

"There is no explanation as to how this huge sum of money was moved into Najib's accounts, the banks used by these Arabs, the sources of the fund, the business of the donor," said Mahathir Mohamad, a former Malaysian prime minister who has become one of Mr Najib's fiercest critics.

Mr Najib, the 62-year-old British-educated son of a former prime minister who has been a close ally of successive Australian governments, has denied any wrongdoing as allegations have swirled around 1MDB - allegations that are now the subject of investigations in Hong Kong, Singapore, Switzerland and the United States.

The Swiss attorney-general's office said last week its investigators had found serious evidence suggesting that  $US4 billion had been misappropriated from Malaysian state-owned companies.

Swiss authorities said some of the money was transferred into bank accounts of former Malaysian officials and former and current officials from the United Arab Emirates.

Singapore said last week it had seized a large amount of bank accounts in connection with its investigations.

Malaysia's opposition MPs have demanded that Mr Apandi prove that 1MDB funds were not used as Mr Najib's political slush fund.

Mr Najib's administration has intensified a crackdown on its critics that has already included sacking the previous attorney-general, who had been investigating the prime minister, and the removal of ruling party members seen as disloyal to Mr Najib.

Mr Apandi said his office is looking to amend the Official Secrets Act 1972 to include life imprisonment and 10 strokes of the cane as punishment for leaking official secrets.

He said journalists who refuse to reveal their sources will be considered as collaborators with a potential saboteur.

"I am serious, no kidding," Mr Apandi told the Sin Chew Daily. "We have too many leakage [sic] of secrets in Malaysia."