Casualty numbers were difficult to gauge but at least several dozen people were killed in the fighting, mainly soldiers. Video footage showed both sides firing rockets and pounding well dug-in positions with heavy artillery, as well as deploying tanks and helicopters.

Alarmed that the fighting could escalate, world leaders urged both sides to sue for peace.

From Washington, John Kerry, US secretary of state, said: “The United States condemns in the strongest terms the large-scale ceasefire violations along the Nagorno-Karabakh Line of Contact, which have resulted in a number of reported casualties, including civilians.”

Russian President Vladimir Putin dispatched Dmitri Medevedev to talk to both Armenian President Serzh Sargsyan in Yerevan and Azerbaijani President Ilham Aliyev in Baku.

As the Bulletin went to press a two-day-old ceasefire held, although there were reports of sporadic fighting.

When the Soviet Union fractured in the late 1980s and early 1990s, localised ethnic tension started to explode into pockets of fighting. Nagorno- Karabakh, a region that belonged to Azerbaijan was one of these region. It was populated mainly by ethnic Armenians who wanted to break away.

After years of fighting that killed 30,000 people the UN negotiated a ceasefire in 1994 that left Armenia- backed rebels running the region.

Thomas de Waal, one of the foremost commentators on the South Caucasus, wrote in the New York Times that the conflict could spread.

“A new all-out Nagorno-Karabakh conflict is the stuff of nightmares. Given the sophisticated weaponry both sides now possess, tens of thousands of young men would most likely lose their lives,” he said.