YEREVAN, Armenia — The Armenian military said Thursday that six of its troops have been taken prisoner by neighboring Azerbaijan, an incident that further exacerbates tensions between the two ex-Soviet nations in the wake of an outbreak of hostilities over the Nagorno-Karabakh region.

Azerbaijan’s Defense Ministry claimed that the Armenian soldiers were seized early Thursday when they attempted to cross the border and place mines on supply routes leading to the Azerbaijani positions.

The Armenian authorities insisted that its soldiers were captured on Armenia’s side of the border while conducting engineering works, and demanded their immediate release.

Armenian Prime Minister Nikol Pashinyan told a government meeting that the soldiers were planting a minefield with warning signs on Armenian territory near the border.

The capture of the Armenian soldiers comes as the South Caucasus neighbors trade angry accusations over the demarcation of their border. Armenia has accused Azerbaijani troops of intrusion and Azerbaijan has denied it.

More than 6,000 people were killed last fall in the six weeks of fighting over Nagorno-Karabakh, which lies within Azerbaijan but was under the control of ethnic Armenian forces backed by Armenia since a separatist war there ended in 1994.

The hostilities ended with a Russia-brokered peace deal in November allowing Azerbaijan to reclaim control over large parts of Nagorno-Karabakh and surrounding areas, which Armenia-backed separatists controlled for more than 25 years.