Amnesty International accused the Sudanese authorities on Wednesday of torturing a British journalist and his translator who were seized in war-torn Darfur while investigating suspected chemical attacks by government forces.

Phil Cox, working for Channel 4 TV, and his Darfuri translator Daoud Hari were abducted in December when they were commissioned by the channel to probe reports by Amnesty that Sudanese security forces had used chemical weapons against civilians in the mountainous Jebel Marra district.

Cox was released in February but on Wednesday Amnesty alleged he and Hari had been tortured while they were in custody.

"For nearly two months, the two journalists were locked up in a prison and tortured, simply for doing their job," Amnesty's Muthoni Wanyeki said in a statement.

"They were beaten, subjected to electric shocks, deliberately deprived of oxygen and subjected to mock executions."

The two men were also chained to a tree in North Darfur when they were detained there, Amnesty said, adding that the two were later moved to Khartoum's Kobar prison.

The journalists' ordeal indicates that the Sudanese authorities have something to hide in Darfur, Amnesty said.

"If no chemical weapons were used, then why not let the journalists get on with their jobs?," Wanyeki said. "This is one more reason, if any were needed, why the alleged chemical attacks which killed an estimated 200 to 250 people must be thoroughly and independently investigated."

Amnesty said in a September report it had credible evidence of Sudanese government forces repeatedly using chemical weapons in Jebel Marra between January and August 2016.

The UN Organisation for the Prohibition of Chemical Weapons said at the time it needed more information and evidence to draw any conclusion based on Amnesty's report.

READ MORE: Sudan accused of chemical weapons attacks in Darfur

Sudanese President Omar al-Bashir, who is wanted by the International Criminal Court on charges of genocide and war crimes in Darfur, has dismissed the Amnesty report as "just empty lies".

Deadly conflict broke out in Darfur in 2003 when ethnic minority groups took up arms against Bashir's government, which responded with counter-insurgency operations.

At least 300,000 people have since been killed and 2.5 million displaced in Darfur, the United Nations says.

"While in Darfur, the two filmmakers were abducted by members of the Rapid Support Forces, who are part of the military and a central element in the Sudanese government efforts to stem the flow of refugees to Europe," Amnesty said.

On Wednesday and Thursday, Channel 4 News is scheduled to air a documentary titled Hunted in Sudan recounting the journalists' ordeal, Amnesty said.