A spokesman told the AFP news agency that officials had been consulting closely with Langdon's lawyers and his employer, US-based contractor Four Horsemen International, about "appropriate representations".

"We understand that his lawyers have lodged an appeal with the Afghan Supreme Court," the spokesman said.

"The case is still subject to appeal, so we do not wish to make any further comment [as it] could jeopardise the man's interests and welfare."

Official intervention

On Wednesday Kevin Rudd, the Australian prime minister, said the government always intervened when Australians were convicted of capital crimes overseas.

"He reached across, and I am ex-military, so it was like bang-bang-bang-bang. I didn't have time to think"

Robert Langdon, Australian on death row for killing Afghan guard

"I don't think it would be wise at this stage to predict or project the effectiveness of any particular intervention by me in the case of this individual. Let's take it step by step," he told reporters.

According to The Australian newspaper, Langdon shot an Afghan colleague, named Karim, four times in the head and body last May as they travelled to a supply convoy, which had been ambushed by the Taliban.

The newspaper said Karim, the Afghan leader of the expedition, reportedly refused to continue when they reached the Wardak provincial capital Maidan Shar, 40km southwest of Kabul, because the road ahead was too dangerous, prompting an argument.

Langdon, who was leading the foreign guards, told the court he pulled his pistol in self-defence after Karim reached for his own firearm.

"He reached across, and I am ex-military, so it was like bang-bang-bang-bang. I didn't have time to think," he said.

Cover-up

But Langdon's self-defence claim was undermined by an admission he had thrown a grenade into the truck with Karim's body in it, and ordered others in the group to fire into the air to fake a Taliban ambush in an attempt to cover-up the incident, the paper said.

The Australian said Langdon was arrested at Kabul airport when he tried to flee to Dubai.

The newspaper also said that Langdon's relatives were trying to raise money for the dead Afghan's family to convince them to allow the Supreme Court in Kabul to spare Langdon's life in line with a local tradition that allows for "mercy payments".

Four Horsemen is a US-based private security and logistics company operating mostly in Afghanistan that employs former American and other troops.

Australia has about 1,500 troops fighting Taliban insurgents and training local security forces in Afghanistan.

US security contractors gained notoriety in Iraq after they killed 17 civilians in Baghdad during a traffic incident in 2007.

A US court last December threw out manslaughter charges against the contractors, who worked for US firm Blackwater Worldwide, but the US government has said it would appeal.

Earlier in January, two US security contractors working for Paravant LLC, a unit of Blackwater's renamed successor Xe, were arrested in Afghanistan on charges they murdered two Afghans in Kabul and wounded a third.