Fighting erupted Tuesday between Armenia and Azerbaijan, the latest flare-up in their decadeslong confrontation in the South Caucasus, killing as many as 100 service members and raising the prospect of fallout from the war in Ukraine spreading instability across a wider region.

Each side blamed the other for the fighting that broke out along their border early Tuesday, in the worst escalation of hostilities between the two countries since a 2020 war and a cease-fire brokered by Russia that ended large-scale fighting.

The resurgence of conflict on Russia’s southern rim — and between two former Soviet republics — raised concerns about what President Vladimir Putin of Russia, already smarting from humiliating defeats in recent days in northeastern Ukraine, might do.