Malaysia's royal rulers have called for a quick, transparent investigation into troubled state-fund 1MDB, in rare comments from the monarchy that presents a new challenge to Prime Minister Najib Razak as he fends off corruption allegations.

1Malaysia Development Berhad (1MDB), whose advisory board is chaired by the prime minister, has amassed debt of more than $11 billion and is at the center of a political storm after allegations of graft and mismanagement.

1MDB has denied wrongdoing. Investigations being conducted by Malaysia's anti-graft agency and other organizations have dragged on with no conclusion in sight.

In a statement, the sultans of nine states and governors of the remaining four said the government's failure to give convincing answers on 1MDB may have resulted in a "crisis of confidence".

"The findings of the investigation must be reported comprehensively and in a transparent manner so that the people will be convinced of the sincerity of the government which shall not at all conceal facts and the truth," the rulers said, according to a report in the Bernama state news agency.

The comments, which come ahead of a two-day gathering of the Conference of Rulers starting on Wednesday, are unprecedented as the sultans assume a non-political role in Malaysia and rarely speak on the functioning of the state.

The remarks add to pressure on Najib, who is facing calls to step down over his alleged involvement in the 1MDB scandal from opposition parties and some establishment figures, including former Prime Minister Mahathir Mohamad.

In July, the Wall Street Journal reported that investigators looking into 1MDB had identified a payment of nearly $700 million into a bank account under Najib's name.

Reuters has not independently verified the report.

Najib has denied taking any money for personal gain and the government has said the deposit was a private donation.

In the weeks that followed, the government replaced the attorney general, who had led investigations into 1MDB, and a parliamentary committee inquiry has stalled.

The royals said they were worried that if the matter dragged on it could jeopardize the economy.

The controversy has battered Malaysian markets with the ring losing a quarter of its value to become Asia's worst-performing currency and Malaysian bonds also down.

"To ensure that the government enjoys the people's trust, that leaders are respected, that political stability is guaranteed and that the economy continues to grow, all leaders must constantly ensure that justice is meted out equitably and transparently based on the law," the rulers said.