Twenty-seven separatist fighters died in clashes with Azerbaijani troops on Monday, officials in Azerbaijan's breakaway Nagorny Karabakh region said, bringing their military death toll to 58.

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.

Karabakh's defence ministry claimed its forces repelled Azerbaijan's tank offensive on the frontline's southern sector.

The Azerbaijani defence ministry said its troops destroyed "several of the enemy's artillery units with a precise strike".

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.

"Twenty-seven servicemen died" in action on Monday, Karabakh's defence ministry said in a statement on the second day of fighting. It has previously put the number of deaths at 28.

The total death toll rose to 67 including nine civilian deaths: seven in Azerbaijan and two on the Armenian side.

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

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.