The shooting down of an Armenian helicopter on the ceasefire line - known as the line of contact that separates Armenian and Azerbaijani forces in the Nagorno Karabakh conflict zone - is the worst incident of its kind in more than 20 years of the truce that ended the war of the early 1990s. Three Armenians were killed, and the Azerbaijani officer who shot down the helicopter was given a medal for courage. It is a very disturbing development that follows a serious upsurge of fighting in the summer in which more than 20 soldiers were killed on both sides, says Thomas de Waal, a senior associate at the Carnegie Endowment for International Peace.

A video taken from the Azerbaijani side shows a missile being fired and one of the two helicopters bursting into flames to shouts of excitement from the Azerbaijani soldiers, the expert notes.

There is no military logic to these attacks. A local commander can be responsible for small arms fire, but use of heavier weapons takes a decision from politicians higher up, de Waal emphasizes, adding that Azerbaijan is pouring hundreds of millions of dollars of oil and gas revenues into buying heavy weapons.

“Meanwhile, the international presence is just as it was in 1994: only six monitors from the Organization for Security and Cooperation in Europe (OSCE) observe the ceasefire along a line of 100 miles (160km), plus the increasingly tense international border between Armenia and Azerbaijan,” he says.

According to de Waal, this violence is a reminder that the two armies are always one step away from another war and that only their own calculation of what is in their best interests holds them back from starting one.

There is every danger now of tit-for-tat retaliations, and the Armenians have already threatened a "painful" response to the downing of the helicopter, he says.