Since the takeover, the majority of Nagorno-Karabakh's ethnic Armenian population has fled and separatist leaders -- including reported billionaire and former official Ruben Vardanyan -- have been detained.

"We strongly condemn the arrests by Azerbaijan of the leaders of Nagorno-Karabakh," Armenia's foreign ministry said in a statement.

"The Republic of Armenia will take all possible steps to protect the rights of the illegally arrested representatives of Nagorno-Karabakh, including in international courts," it said.

Separatist authorities in the breakaway region agreed to disarm, dissolve their government and reintegrate with Azerbaijan in the wake of Baku's one-day military operation in late September.

Azerbaijan's Prosecutor General Kamran Aliyev said criminal investigations had been initiated into crimes committed by 300 separatist officials.

But the United Nations human rights office has urged Baku to afford Vardanyan and other detainees "full respect and protection".