Twenty-six separatist fighters died in clashes with Azerbaijani troops late on Monday in Azerbaijan's breakaway Nagorny Karabakh region, the rebels' defence ministry said, bringing their military death toll to 84.

World leaders have urged a halt in fighting after the worst escalation since 2016 raised the spectre of a fresh war between the ex-Soviet rivals Armenia and Azerbaijan.

Fierce clashes continued throughout the day on Monday, officials in Baku and Yerevan said.

On Monday evening, Azerbaijani forces launched a "massive offensive at the Karabakh frontline's southern and north-eastern sectors," said Armenia's defence ministry spokesman Artsrun Hovhannisyan.

"Twenty-six servicemen of Karabakh's Defence Army died" in action, the Karabakh's defence ministry said in a statement late on Monday.

Armenia and Azerbaijan have been locked in a territorial dispute since the 1990s when Karabakh declared independence after a war that claimed 30,000 lives.

No country recognises Karabakh's independence -- not even Armenia -- and it is still considered part of Azerbaijan by the international community.

The total death toll rose to 95 including 11 civilian deaths: nine in Azerbaijan and two on the Armenian side.

Azerbaijan has not yet released information on military casualties since the latest fighting broke out.

Fighting between Muslim Azerbaijan and majority-Christian Armenia could embroil regional players, Russia and Turkey.

Russia, which has a military alliance with Armenia, sells sophisticated weapons worth billions of dollars to both Baku and Yerevan.

Armenia has accused Turkey -- which backs Turkic-speaking Azerbaijan -- of meddling in the conflict.

Talks to resolve one of the worst conflicts to emerge from the 1991 collapse of the Soviet Union largely stalled in 1994 when a ceasefire was agreed.

France, Russia and the United States have mediated peace efforts as the "Minsk Group" but the last big push for a peace deal collapsed in 2010.