Armenians have protested after reports that as many as 100 soldiers were seized by Azerbaijani forces in Nagorno-Karabakh.
They were captured weeks after a war in which at least 5,000 servicemen died and Azerbaijan made territorial gains.
Karabakh is internationally recognised as part of Azerbaijan but is under Armenian control.
The recent conflict was brought to an end by a Russian-mediated peace deal, but clashes have broken out again.
As part of the agreement, Armenia handed three areas over to Azerbaijan and the flare-up took pace in one of them, a southern area of Karabakh.
The exact number of captured soldiers is unclear but reports range from 60 to as many as 160, seized as part of an Azerbaijani "anti-terror" operation. Several videos posted online appeared to show captured soldiers.
On Tuesday night, Karabakh's defence ministry said contact had been lost with a number of military posts. The defence ministry in Azerbaijan has refused to discuss the matter with the BBC.
Anger spread as videos emerged on social media in Azerbaijan, and Nagorno-Karabakh human rights ombudsman Artak Beglaryan said it was highly likely that captive Armenian soldiers were featured in the footage. He put the number missing at around 60.
Protests took place in Armenia's capital, Yerevan, and in Karabakh itself.
Families of the missing men blocked a main road demanding to know more details and protesters marched on the defence ministry in Yerevan.
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The Azerbaijan operation began at the end of last week. Armenia accused Azerbaijan of breaking the November peace deal by attacking the two villages called Hin Tagher (Kohne Taglar in Azerbaijani) and Khtsaberd (Calakkala).
Azerbaijan says the two villages fall under its control under the terms of the peace deal and that it launched the offensive to tackle Armenian servicemen who had refused to leave the area after the truce.
Four Azerbaijani soldiers were killed at the weekend, the first casualties since the war came to an end on 10 November.
Although 2,000 Russian peacekeepers have been deployed between the two sides, Armenian reports said they were not covering the area where clashes had taken place.
When Russia's defence ministry published a map extending its peacekeepers' deployment to cover the area, Azerbaijan objected.
Since the war ended in November, Armenia has handed over three areas lost in the war to Azerbaijan under a peace deal.
The two sides also began exchanging prisoners of war this week, with Armenian Prime Minister Nikol Pashinyan promising that 44 servicemen would return home.
In a separate development, Azerbaijan said it had charged two of its soldiers with mutilating the bodies of Armenian soldiers during the war.