Former deputy premier Musa Hitam has told Prime Minister Najib Razak to decisively answer the allegations against him and his administration, adding heft to mounting criticisms against Datuk Seri Najib.

In speaking out, Tun Musa joins another respected Umno elder, Tun Dr Mahathir Mohamad, who has been putting pressure on Mr Najib to come clean.

Mr Musa, in an interview with The Malaysian Insider news website, said Mr Najib must answer "questions and doubts" raised by his political opponents, and not "make the accusation that this was done to topple his administration".

Two days ago, he urged Mr Najib to take a leave of absence to give investigators from a multi-agency government task force freedom to carry out a credible probe.

Mr Najib's bank accounts - two of which have been closed - are under scrutiny following allegations in the Wall Street Journal (WSJ) newspaper early this month that US$700 million (S$946 million) were deposited into them between March 2013 and February this year.

The funds are believed to be linked to state investor 1Malaysia Development Berhad (1MDB), whose chief adviser is Mr Najib.

In the Insider interview, Mr Musa said previous prime ministers might have survived similar scandals as they were not running governments in a digital democracy.

"They managed to survive but I am sorry to tell Najib that this cannot work as times have changed."

While Umno does not have a formal or informal grouping of elders who act as advisers, the views of respected elder statesmen like Dr Mahathir and Mr Musa carry weight in the party and among the majority Malays. Others who are considered Umno greybeards include Mr Najib's predecessor Abdullah Badawi, former finance minister Tengku Razaleigh Hamzah and former foreign minister Tengku Ahmad Rithauddeen, who leads Umno's disciplinary committee.

The three men have yet to comment on the WSJ allegations.

On the same day as the WSJ report on July 3, veteran opposition leader Lim Kit Siang suggested in his blog that Mr Najib should step down temporarily while "the three Tuns" help run the country.

Mr Najib's answer was to set up a multi-agency task force to look into the allegations. The task force is headed by four people appointed by him, including central bank governor Zeti Akhtar Aziz.

This has raised questions whether the task force members could find their boss guilty.

Asked about the task force's credibility, Youth and Sports Minister Khairy Jamaluddin said: "I do not think she (Dr Zeti) or the others would want to put (their) reputation on the line by overseeing an investigation which is not credible."