Scores of soldiers were killed in heavy clashes between Armenian and Azerbaijani forces despite calls for a ceasefire. A pro-Armenian separatist leader said he was heading into a “final battle” in Nagorno-Karabakh.

Armenian and Azerbaijani armed forces were locked in intense warfare over the disputed Nagorno-Karabakh province on Saturday, with Armenia reporting that over 50 pro-Armenian separatist soldiers lost their lives.

Armenia-backed separatist forces had thwarted a “massive attack” by Azerbaijan and begun a counter-offensive, Armenian defense ministry spokeswoman Shushan Stepanyan said. “Heavy fighting is ongoing,” she added.

Read more: Nagorno-Karabakh: Armenia ‘stands ready’ for peace talks

Pro-Armenian Karabakh separatist leader Arayik Harutyunyan said he was on his way to the front line for what he described as the “final battle.”

Read more: US, Russia, France condemn fighting in Nagorno-Karabakh

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

Both Armenia and Azerbaijan have defied international calls for a ceasefire while accusing each other of starting the recent conflict.

The fighting, which broke out last Sunday, has seen the heaviest clashes since a 1994 truce between the warring sides. Some 200 people have perished, including more than 30 civilians.

The conflict increases the likelihood of a wider regional war that could involve Russia and Turkey amid concern about stability in the South Caucasus, where pipelines deliver Azeri oil and gas to global markets.