Azerbaijan has admitted mistakenly shooting down a Russian Mi-24 military helicopter over Armenia, killing two crew members and injuring a third.

In a statement, Azerbaijan apologised, offering to pay compensation.

Russia said a missile downed its Mi-24 accompanying a column of Russian troops in Armenia, where Russia has a base. It comes amid fierce fighting between Azerbaijan and Armenian separatists for Nagorno-Karabakh. But the helicopter was not downed near those clashes.

Nagorno-Karabakh is internationally recognised as part of Azerbaijan, but has been run by ethnic Armenians since 1994.

In the statement, Azerbaijan’s foreign ministry said the Russian Mi-24 was shot down at 18:30 local time « on the Nakhchivan section of the Armenia-Azerbaijan state border ».

The Nakhchivan Autonomous Republic is Azerbaijan’s western landlocked exclave, bordering Armenia, Iran and Turkey.

Azerbaijan’s foreign ministry said the helicopter had been flying at a low altitude during hours of darkness, and outside « the air defence radar detection zone ». It said that « Russian Air Force helicopters have not been seen in the specified area before ». In the context of these and other factors, « the combat crew on duty took the decision to open fire, » the statement said.

The Russian defence ministry said its Mi-24 was shot down by a missile as it was accompanying a column of vehicles belonging to the 102nd Russian military base. It said this happened near the town of Yeraskh. The helicopter lost control and crashed in a mountainous area on Armenian territory.

The Russia foreign ministry later tweeted: « We regard positively the fact that Baku recognised that it was to blame without delay. » It also said that Azerbaijan had pledged to carry out a speedy investigation into the incident and those responsible would be punished.

Russia has a military base in Armenia, and the two countries are also members of the Moscow-led Collective Security Treaty Organization.

The treaty envisages Russia’s military support if Armenia is attacked – but it does not include Nagorno-Karabakh or the other Azerbaijani regions around it seized by Armenian forces. At the same time, Moscow also has strong ties to Azerbaijan, which is being openly backed by Turkey, a Nato member. In addition, Russia has been selling weapons to both Armenia and Azerbaijan.