Two opposition parties have demanded an explanation from Defence Minister Lindiwe Sisulu for the sudden resignation of two senior defence officials.

The resignations of South African Air Force chief Lt-Gen Carlo Gagiano and the Secretary of Defence Mpumi Mpofu raised serious questions, said the Democratic Alliance (DA) and the Freedom Front Plus.

DA defence spokesperson David Maynier said the reasons for the resignations "remain murky". Sisulu could not allow speculation on the matter to continue.

"The resignations create a serious leadership vacuum at a time when the defence force has begun a defence review to reset its mandate capabilities and funding," Maynier said.

FFPlus MP Pieter Groenewald described the resignations as rash.

"It appears as if both are being held responsible for circumstances over which they had no control," he said

The resignations have been linked to a string of incidents involving the transport of Deputy President Kgalema Motlanthe.

Last month, a VIP plane chartered by the air force developed technical problems as it was taking off from Waterkloof Air Force Base in Pretoria to fly Motlanthe to Finland for an official visit.

In 2009, Motlanthe's plane had to make an emergency landing while flying back from Libya.

In September, the plane flying him to the opening of the Rugby World Cup in New Zealand missed its first landing slot "as a precautionary measure".

In another case, two pilots who flew President Jacob Zuma to the United States were found to have been involved in the failed 2004 coup in Equatorial Guinea.

Groenewald said it was clear that Gagiano and Mpofu were being held responsible for the embarrassment of the president and his deputy with rented aircraft.

"Lt-Gen Gagiano has to manage an air force where the government does not even give sufficient funding for fuel for pilots to be able to fly the required minimum number of flight hours. The impossible is expected of him."

Groenewald said Mpofu had wanted to ensure that the correct acquisition procedures were followed when buying new military aircraft.

"She impressed as the new secretary of defence and it is reflected in the auditor general's report. Both these individuals are assets to the defence force and should be kept."

A department spokesperson confirmed on Friday that Mpofu and Gagiano had resigned with immediate effect.

According to the Cape Times newspaper Sisulu refused to accept Gagiano's letter, but the minister lost her patience when Mpofu failed to give satisfactory answers over the debacle.

Mpofu has been replaced by the chief financial officer Mziwonke Dlabantu.