By Skye Wheeler

JUBA, Sudan, Aug 4 (Reuters) - South Sudan's parliament has ordered an investigation into a government minister it says has delayed the prosecution of senior officials accused of corruption in the semi-autonomous oil-rich region.

Parliament summoned South Sudan's Interior Minister Paul Mayom this week to explain why a probe into allegations that finance ministry officials paid vastly inflated prices for government and police vehicles was progressing so slowly.

The case is a high-profile test of the "zero tolerance" policy on corruption declared by South Sudan President Salva Kiir, who took office after a January 2005 north-south peace deal ended more than two decades of civil war.

Member of parliament Peter Bandi told Reuters Mayom had dismissed the chief investigator in the case for reasons that were unclear, and that Mayom's answers to parliament on Wednesday had been unsatisfactory.

"He is supposed to be administering justice. If you become an obstruction to the execution of justice, our security is not in safe hands," said Bandi.

In March, Kiir stripped Finance Minister Arthur Akuein Chol of his immunity from prosecution to allow investigators to question him in connection with the purchase of cars from the Al Cardinal company. Chol is currently on bail.

Bills for the vehicles showed they had been bought for $95,000 each, compared with what investigators said was a market value of between $40,000 and $45,000.

Bandi said MPs had seen substantial evidence, including correspondence between Mayom and the junior state minister for the interior in the Sudanese capital Khartoum, that Mayom had been trying to obstruct justice in the case.

Mayom, whose role includes administering the police service, is also suspected of trying to transfer the chief prosecuting attorney out of south Sudan's capital Juba, where court cases are supposed to take place.

But the reasons for Mayom's behaviour and the exact role of the state minister are still unclear, said Bandi.

DENIAL

Mayom denied the allegations late on Friday, telling Reuters he had recommended the chief investigator be removed after discovering he was under suspicion for illegally taking extra cash while working for a governmental institution in north Sudan.

"(We) have not issued an order for dismissal, not at this end," said Mayom.

Mayom also said the planned transfer of the chief prosecutor was a decision made and then withdrawn by the legal affairs ministry, not by his office.

The government spent over $70 million on four-wheel-drive vehicles from the Al Cardinal company, but not all the vehicles were delivered, said Bandi.

"We were supposed to receive 490-something cars but what came was 158," said Bandi. The finance ministry had imposed the deal on other ministries, he said.

After Chol's ministerial immunity was suspended in March, his temporary replacement announced that some $498 million of reserves had been spent without parliamentary approval.

Then-Acting Finance Minister Gabriel Changson Chang called for government spending agents to tighten their belts in light of smaller than expected oil revenues, on which the south is almost entirely reliant.

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