Malaysia's prime minister on Thursday dismissed as "rubbish" a former police commando's claim that he was ordered by "important people" to kill a woman linked to highly sensitive corruption allegations.

"It's total rubbish. Total rubbish," Prime Minister Najib Razak, in a rare comment on the affair, said in a brief remark to reporters, according to news website Malaysian Insider.

Policeman Sirul Azhar Umar, who fled abroad to avoid being hanged and is now in Australian custody, is a key figure in a scandal entwined with Malaysia's 2002 purchase of submarines from France.

That deal has long been clouded by accusations of huge kickbacks to Malaysian officials and the murder of Altantuya Shaariibuu, a Mongolian woman who purportedly acted as a translator in the negotiations.

Suspicions have swirled for years that Altantuya was murdered to keep her quiet about shenanigans in the deal.

In a phone interview published Wednesday by Malaysian news website Malaysiakini, Sirul said he and another officer carried out the killing under orders from "important people", while declining to elaborate.

Najib has previously denied wrongdoing in the deal and sworn on the Koran that he did not know Altantuya.

The scandal is one of Malaysia's most sensitive topics.

It centres on allegations that French submarine maker DCNS paid "commissions" of more than 114 million euros ($142 million) for two Scorpene submarines, which Malaysia's opposition alleges were kickbacks.

The deal is being investigated by French authorities.

Altantuya was the lover of Abdul Razak Baginda, a close Najib associate tasked with executing the deal. Najib was defence minister at the time.

Altantuya, 28, was shot dead and her body blown up with military-grade plastic explosives outside Kuala Lumpur.

A private investigator working for Abdul Razak gave a sworn 2008 statement claiming Najib, now 61, had earlier been romantically entangled with Altantuya and was involved in efforts to manipulate the murder probe.

Sirul and his convicted accomplice Azilah Hadri were members of an elite unit that guards top Malaysian ministers.

They were convicted of the killing in 2009 and sentenced to death, a verdict upheld by Malaysia's highest court last month. Sirul had managed to flee abroad ahead of last month's final ruling.

But Malaysia's courts never sought to establish why two police commandos would act on their own in killing the woman, fuelling suspicions of a cover-up.

Sirul told Malaysiakini he was a "scapegoat" and was considering revealing all he knows to the media.

He is now stuck in limbo in custody in Australia, where national law forbids sending people back to face execution.

Sirul's flight abroad has revived calls in Malaysia for a thorough re-examination of the affair.

Authorities have so far dismissed that possibility, with the hardline police chief threatening that anyone who questions the outcome of the murder case could face contempt-of-court charges.