The Kremlin, as the former colonial ruler, has presented itself as a mediator between the two, while also selling arms to both. Russia also maintains a small base in Armenia. On Saturday, President Vladimir V. Putin of Russia called for a cease-fire.

Analysts were struggling to understand what had caused this eruption and whether it indicated the start of a new, violent phase of the war. The ethnic war that began in the late Soviet period claimed more than 20,000 lives and ended in the cease-fire, but there was no final settlement.

The former Soviet Union is dotted with at least five frozen conflicts that Moscow occasionally heats up to exert pressure on independent states it once controlled, including Georgia, Moldova and most recently Ukraine. Russia does not have a proxy force in the Nagorno-Karabakh conflict, as it does in the others, but both the Armenian government in Yerevan and the Azeri government in Baku depend on Moscow to referee the standoff.

Some analysts described the recent fighting as a natural outburst of the tensions that build up along the cease-fire line but that in this case escalated markedly, not least because both sides were deploying far more sophisticated weaponry.

Instead of just exchanging mortar fire, for example, there were reports that the countries were deploying heavy weapons for the first time since 1994, with the two sides lobbing Grad rockets at each other, which cause far more extensive and unpredictable damage.

“A provocation that begins with the use of large-caliber multiple rocket launch systems and gunships has significantly higher chances of leading to an accidental war because of the casualties it can cause,” wrote Simon Saradzhyan, a Russia specialist at Harvard Kennedy School’s Belfer Center for Science and International Affairs, in a preliminary online analysis.

Azerbaijan alone in recent years has used its oil wealth to purchase about $4 billion worth of new, mostly Russian weapons. President Ilham Aliyev faced public pressure to show something for the investment, especially amid growing public unrest after the collapse in global oil prices. The country’s currency, the manat, dropped about one-third in value against the dollar in December.