The European Union called Sunday for a halt to fighting between Armenian and Azerbaijani separatists over the Nagorny Karabakh region and an "immediate return to negotiations".
The worst clashes since 2016 broke out on Sunday between arch-foes Azerbaijan and Armenia, which have been locked for decades in a territorial dispute over the Armenia-backed breakaway region.
"Military action must stop, as a matter of urgency, to prevent a further escalation," European Council president Charles Michel tweeted, calling for "an immediate return to negotiations, without preconditions".
EU foreign policy chief Josep Borrell stressed the urgency of relaunching negotiations over Nagorny Karabakh under the auspices of the so-called "Minsk Group" led by France, Russia and the United States.
"The European Union calls for an immediate cessation of hostilities, de-escalation and for strict observance of the ceasefire," he said.
A major conflict between Azerbaijan and Armenia could draw in top regional players Russia and Turkey.
The conflict over Nagorny Karabakh has been fuelling regional tensions for 30 years.
Ethnic Armenian separatists seized the region from Baku in a 1990s war that claimed 30,000 lives.
Since then, the Azerbaijani authorities have sought to regain control of it, by force if necessary.
Peace talks have been largely stalled since a 1994 ceasefire agreement. The last big push for a peace deal collapsed in 2010.