Both sides reported shelling as violence erupted in the territory, over which Armenia and Azerbaijan fought a 44-day war in 2020.
Azerbaijan's Defence Ministry said it was firing at military targets as part of “anti-terrorist activities” to disarm Armenian forces in the region. Nagorno-Karabakh is internationally recognised as part of Azerbaijan, but the majority of people there are ethnically Armenian.
Armenia said its neighbour had “unleashed another large-scale aggression against the people of Nagorno-Karabakh, aiming to complete its policy of ethnic cleansing”. A separatist, pro-Armenian government in the region called it a “large-scale military offensive” by Azerbaijan and reported civilian casualties.
Blasts were heard in the separatist stronghold of Stepanakert, AFP reported, as Azerbaijan said it was firing “high-precision weapons” against military targets. Armenia rejected Azerbaijan’s claim that units of its military were in Nagorno-Karabakh.
Six people were reported killed in landmine explosions hours before Azerbaijan announced its offensive.
Josep Borrell, the EU's top diplomat, called for the “immediate cessation of hostilities and for Azerbaijan to stop the current military activities”.
“There is an urgent need to return to dialogue … this military escalation should not be used as a pretext to force the exodus of the local population,” Mr Borrell said. European Council president Charles Michel called the offensive “devastating news”.
Russian Foreign Ministry spokeswoman Maria Zakharova said Moscow was “deeply concerned” about the escalation after being informed about Azerbaijan's operation “only minutes before it began”.
The territory has been disputed since the 1990s aftermath of the break-up of the Soviet Union. Azerbaijan regained parts of Nagorno-Karabakh and surrounding areas in the six-week war in 2020, which ended with an armistice.
Azerbaijan alleges that Armenia has smuggled in weapons since then. Tension has simmered over a blockade of the road connecting Nagorno-Karabakh to Armenia, causing severe food and medicine shortages in the region.