A number of Armenian soldiers have been killed and captured in a flare-up of violence on the border with Azerbaijan, Armenian officials say.
Armenia said some of its troops had been killed and two combat positions had been lost, while Azerbaijan said two of its soldiers were injured.
Later on Tuesday, both side reportedly agreed to a Russian-brokered ceasefire.
More than 6,000 lives were lost in last year's war over the disputed territory of Nagorno-Karabakh.
Azerbaijani forces, backed by Turkey, managed to recapture large swathes of the region of what is internationally regarded as part of Azerbaijan.
On Tuesday, Armenia asked Russia, a key security ally with long-standing ties to the former Soviet republic, to help defend its territorial sovereignty against Azerbaijan.
Both the Armenian and the Russian defence ministries later said that the Russian-backed truce was agreed.
Azerbaijan has not publicly commented on the issue.
Earlier, Armenia blamed Azerbaijani troops for the latest outbreak of fighting and said 12 soldiers had been captured.
It did not immediately confirm details of casualties – but the head of parliament's foreign relations committee, Eduard Aghajanyan, said as many as 15 soldiers may have died.
Azerbaijan's defence ministry accused Armenia of "large-scale provocations against Azerbaijan in the Kalbajar and Lachin regions of the state border"
It said Armenia "launched a sudden military operation" to "take more advantageous positions" – but that the attack had failed.
But according to Armenia's foreign ministry, Azerbaijani forces attacked the eastern border as part of a policy that began in May aimed at infiltrating two Armenian areas – Syunik in the south-east and Gegharkunik in the east.
By Konul Khalilova, Editor, BBC Azerbaijani
A year after they fought a 44-day war, these latest tensions are threatening the peace deal between Armenia and Azerbaijan.
The single biggest issue is that Azerbaijan wants to have a corridor running through to its exclave Nakhichevan – and ultimately to its ally Turkey. This so-called Zangazur corridor would have to pass through Armenian territory, but Armenia will not accept having a road on its land under Azerbaijan's control.
Politically it is important for Azerbaijan's leader, Ilham Aliyev, who pledged earlier this year that he would "force" Armenia to concessions regarding the corridor.
Neither country appears committed to the peace deal and both are increasing their military budgets. However for the moment at least, diplomatic contacts between them have not broken down.
Responding to Tuesday's border clashes, European Council President Charles Michel urged the leaders of Armenia and Azerbaijan to negotiate a "full ceasefire".
Mr Michel said he called for an "urgent de-escalation" in discussions with President Aliyev and Armenia's Prime Minister Nikol Pashinyan.
A Russian-brokered peace deal was reached in November 2020. Some 2,000 Russian peacekeepers were deployed to patrol the area in and around Nagorno-Karabakh.
- Why did Nagorno-Karabakh spark a conflict?
- Karabakh war leaves civilians shell-shocked and bitter
During the dying years of the USSR, Armenia was drawn into a bloody conflict with Azerbaijan over the mainly ethnic-Armenian Nagorno-Karabakh region.
Armenia is majority Christian while Azerbaijan is majority Muslim. Turkey has close ties to Azerbaijan, while Russia is allied with Armenia.