Armenia on Tuesday reported deaths and the loss of military positions in border clashes with Azerbaijani troops, a year after the arch-foes fought a war over the disputed Nagorno-Karabakh region.
The six-week conflict, which left more than 6,500 dead, ended last November in a Russian-brokered deal that saw Armenia cede swathes of territory it had controlled for decades.
Armenia's defence ministry said on Tuesday that "there are fatalities and wounded among Armenian troops as a result of fighting that erupted following an attack by Azerbaijani forces."
It said the number of casualties was being verified and that Armenia had "lost control of two military positions."
The ministry reported later in the evening that 12 Armenian servicemen were captured by the Azerbaijani military.
Earlier on Tuesday, the two sides accused each other of initiating fighting along their shared border.
"Armenian troops attacked Azerbaijani positions in the districts of Kelbajar and Lachin," Azerbaijan's defence ministry said in a statement, adding that two Azerbaijani troops were wounded.
The ministry said Azerbaijani troops "stopped the enemy's advance, surrounded and detained Armenian servicemen."
The country's foreign ministry said Armenia "is deliberately mounting tensions and is not interested in ensuring peace and security in the region".
Armenia's defence ministry said Azerbaijani forces tried to "break through" the border before being repelled.
- Appeals to Moscow -
The European Union called on both sides to cease hostilities.
"Call for urgent de-escalation and full ceasefire," European Council President Charles Michel said on Twitter.
"Challenging situation in region -- EU is committed to work with partners to overcome tensions for a prosperous and stable South Caucasus," he added.
Armenia appealed to ally Russia for military support under the Collective Security Treaty Organisation pact, which obliges Moscow to protect it in the event of a foreign invasion.
"Given that there was an attack on Armenia's sovereign territory, we appeal to the Russian Federation to protect Armenia's territorial integrity," said Security Council Secretary Armen Grigoryan.
Since last year's war, both Armenia and Azerbaijan have reported occasional exchanges of fire, sparking fears of another flare-up in their territorial dispute.
Both Armenia and Azerbaijan said the situation along their border remained tense with clashes ongoing on Tuesday evening.
On Sunday, they traded accusations of opening fire at their border near Karabakh.
The day before, Nagorno-Karabakh authorities said the only road connecting Armenia to the separatist territory -- the Lachin Corridor -- was briefly closed due to an incident between the two sides.
Ethnic Armenian separatists in Nagorno-Karabakh broke away from Azerbaijan as the Soviet Union collapsed in 1991, and the ensuing conflict claimed around 30,000 lives.