Fighting between Armenia and Azerbaijan on Monday night killed almost 100 soldiers, with Armenia accusing Azerbaijan of acts of aggression.
Armenia's Prime Minister Nikol Pashinyan said 49 of its soldiers died in overnight clashes, while Azerbaijan's defence ministry said 50 of its servicemen were also killed.
At the core of the dispute is the region of Nagorno-Karabakh. It is, according to internationally-recognised borders, firmly a part of Azerbaijan - but is populated by ethnic Armenians.
The dispute has led to full-scale war in the 1980s and 1990s, a six-week war in 2020 and continuing clashes for decades.
The two countries put blame on each other for the latest outbreak of violence. Armenia has claimed that several towns along the border had been shelled and it responded to the acts of aggression, whilst Azerbaijan said its infrastructure came under attack first.
Monday night's fighting is believed to be the worst since the 2020 conflict, in which thousands were killed. That war ended in a deal brokered by Russia, which saw Armenia withdraw its troops from occupied areas around Nagorno-Karabakh.
The Secretary general of the UN called on "the sides to take immediate steps to deescalate tensions, exercise maximum restraint & resolve any outstanding issues through dialogue and within existing formats". He also urges them to fully implement previously reached agreements.
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