Armenian soldiers and a Russian peacekeeping soldier, on the vehicle, at a checkpoint in the disputed area of Nagorno-Karabakh in 2020.Credit...Mauricio Lima for The New York Times

Fighting erupted Tuesday between Armenia and Azerbaijan, the latest flare-up in their decades-long armed standoff over the disputed region of Nagorno-Karabakh in the South Caucasus. The clashes raised the prospect of Russia becoming embroiled in another war near its borders.

Each side blamed the other for the fighting that broke out along their border early Tuesday. It was the worst escalation of hostilities between the two countries since a 2020 war over Nagorno-Karabakh and a cease-fire brokered by Russia that ended large-scale fighting between Armenian and Azerbaijani forces.

Prime Minister Nikol Pashinyan of Armenia said that 49 of his country’s servicemen had died overnight in clashes with the Azerbaijani Army. Azerbaijan’s defense ministry accused Armenia of a number of “large-scale provocations” that forced it to retaliate. The ministry later announced that 50 Azerbaijani service members — 42 soldiers and eight border guards — had been killed.

Mr. Pashinyan, speaking in the Armenian Parliament, denied that his country had provoked Azerbaijan, and accused it of attacking Armenian territory first. He said that the intensity of hostilities had decreased later Tuesday, but that attacks from Azerbaijan were continuing on one or two fronts.

Russia is a close ally of Armenia, and some military analysts suggested that Azerbaijan may have been emboldened by Russia’s recent setbacks in northeastern Ukraine to launch a new attack. Nagorno-Karabakh is internationally recognized as part of Azerbaijan but claims independence and is closely allied with Armenia.

Mr. Pashinyan appealed to Russia, and President Vladimir V. Putin, with whom he talked by phone, to resolve the situation, according to an Armenian government statement.