Azerbaijan says it has destroyed an Armenian air defence missile system in the breakaway Nagorno-Karabakh region, drawing a sharp response from the separatists who vowed retaliation.

Azerbaijan and Armenia - part of the former Soviet Union until 1991 - are locked in a protracted conflict over the disputed region, and almost went to war in April 2016 following deadly border clashes.

"Azerbaijani forces destroyed on Monday an Armenian Osa air defence system and its crew in the Fisuli-Khojavend sector of Karabakh's frontline in order to avert the threat it posed to Azerbaijan's aircraft," an official from the press service of Azerbaijan's defence ministry told the AFP news agency on Tuesday.

The Nagorno-Karabakh separatist authorities confirmed that an Azerbaijani attack damaged its equipment, but said there were no casualties. It warned in a statement that the Azerbaijani "provocation won't be left unanswered".

"Azerbaijani forces' provocation will not be left unanswered," it said.

The incident comes months after several Azerbaijani servicemen were killed in February in with Karabakh troops.

In April 2016, at least 110 people from both sides were killed as simmering violence flared into the worst fighting in decades.

A Russian-brokered ceasefire ended the four days of fierce clashes but attempts to relaunch the stalled peace process since then have failed.

OPINION: Nagorno-Karabakh is not a localised conflict

Nagorno-Karabakh is officially part of Azerbaijan, but since a separatist war ended in 1994 it has been under the control of forces that claim to be local ethnic Armenians - Azerbaijan claims they include regular Armenian military.

That war in the early 1990s claimed some 30,000 lives, and the two sides have never signed a firm peace deal.

Russia wields influence in the region and has sponsored mediation to end past clashes. It has stationed thousands of its troops and military hardware in Armenia, a close ally.