BAKU, Azerbaijan (AP) — A sharp escalation in fighting between Azerbaijan and Armenia over the disputed territory of Nagorno-Karabakh has left 15 soldiers dead and prompted Russia to issue an urgent call for calm.

Azerbaijan’s Defence Ministry said yesterday that 12 of its troops have been killed in the past four days, including four overnight. Nagorno-Karabakh’s armed forces said one of its soldiers was killed early yesterday, the third in recent days.

Azerbaijan’s Nagorno-Karabakh region and some adjacent territory have been under the control of Armenian soldiers and ethnic Armenian local troops since the end of a six-year separatist war in 1994.

Both sides report frequent shootings and attempted incursions along the ceasefire line, but the latest outbreak of fighting is the worst in many years. It was not immediately clear what set off the latest violence between the former Soviet republics, with Azerbaijan and Armenia each accusing the other of being the aggressor and claiming to have repelled a series of attacks.

Russia’s annexation of Crimea, however, has contributed to the tensions. Armenia, which depends on Russia for economic and military support, has welcomed the takeover of Crimea and some Armenians have suggested it could be a model for Nagorno-Karabakh. This has rattled Azerbaijan, which like Ukraine has aligned itself with the West.