MOSCOW — Fighting that was reported to be fierce broke out on Sunday between Azerbaijan and Armenia and quickly escalated with the two sides claiming action with artillery, helicopter and tanks along a disputed border.
The military action centered on the breakaway province of Nagorno-Karabakh, a mountainous area north of Turkey and Iran where ethnic tensions and historical grievances have made kindling for conflict for decades.
The fighting on Sunday, however, was reportedly more severe than the typical periodic border skirmishes, and both governments used military language describing the events as war. Before Sunday, the last major escalation was in 2016. Each sides accused the other of using artillery against civilians.
“The enemy has started an attack” on the Karabakh region, the Armenian prime minister, Nikol Pashinyan, said in a post on Facebook.
Mr. Pashinyan said the military of the Karabkah region, an ethnic Armenian enclave that claims to be an independent state but is mostly unrecognized, had repelled the attack.
But the Ministry of Defense of Azerbaijan then issued a statement saying it had begun a “counterattack” with tanks, helicopters and rocket artillery.
In a statement carried by Russian news agencies, Azerbaijan said the military operation had destroyed “troops, military objects and equipment of the Armenian armed forces” near the border as well as deeper inside the country. It said it destroyed 12 short-range antiaircraft installations in Armenia.
The Armenian Defense Ministry said its forces had destroyed three tanks and shot down two helicopters, Reuters reported.
In past flare-ups, both sides have exaggerated their successes and the scale of their enemies’ violations of cease-fire agreements, though the potential for a wider war has always been clear. The Karabakh region maintains a system to call up nearly its entire male population as minutemen, and this mobilization was announced Sunday morning.
Fighting in and around the Karabakh region, which Armenia calls Artsakh, was among the most vicious of the early post-Soviet conflicts. A cease-fire was declared in 1994, but violence has often flared up since.
Moscow sells weapons to both sides and has also brokered cease-fire agreements. Russia has a military base in Armenia. The Armenian diaspora in France and the United States has aided the Karabakh region, including financing construction of a strategic mountain-access road.