Baku and Yerevan have for decades been locked in a simmering conflict over Nagorno-Karabakh, an ethnically Armenian region of Azerbaijan which broke away from Baku in a 1990s war that claimed the lives of some 30,000 people.

Both sides have defied international calls for a ceasefire and accused the other of starting the new clashes that began last Sunday and have seen the heaviest fighting since a 1994 ceasefire.

On Saturday, Armenia-backed separatist forces repelled a "massive attack" by Azerbaijan and launched a counter-offensive, Armenian defence ministry spokeswoman Shushan Stepanyan said.

"Heavy fighting is ongoing on other flanks," she wrote on Facebook.

Karabakh separatist leader Arayik Harutyunyan said a "final battle" was under way with Azerbaijani troops.

"The nation and motherland are under threat. Time has come for the entire nation to become a powerful army," he told journalists before joining troops at the battlefield.

Karabakh army spokesman Suren Sarumyan said Azerbaijani forces who were using aviation, drones, and tanks met "heroic resistance" from separatist fighters.

The Azerbaijani defence ministry claimed its forces had "captured new footholds (in Karabakh) and cleaned up the territory from enemy troops."

Nearly 200 people have been confirmed killed since the fighting erupted last Sunday, including more than 30 civilians.

There are fears the fighting could expand into an all-out, multi-front war involving regional powers Turkey and Russia.

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