Armenian President Serzh Sargsyan has called for a ceasefire, as an outbreak of violence in breakaway Nagorno-Karabakh continues. Armenia and Azerbaijan have been locked in a bitter dispute over the mountainous region. Sargsyan also says the Organization for Security and Co-operation in Europe should start an investigation on the situation.

"Armenia and Nagorno-Karabakh stand for a cessation of military actions and for strict observance of the ceasefire regime declared in 1994, as well as for the return of all military units to positions and bases which they occupied before April 1st 2016."

"In order for the ceasefire to be observed, the OSCE should come up with steps aimed at stabilising the situation and the current conditions. They are: First of all, urgent investigation of ceasefire violations and a significant increase of the monitoring potential of the personal representative of the current OSCE chairman. It is also necessary to increase the number of field assistants of the personal representative of the current OSCE chairman which will monitor ceasefire violations," Serzh Sargsyan said.

Despite calls from international mediators for an immediate halt to the fighting, the two sides exchanged artillery fire and reported clashes in the region on Monday. Azerbaijan's defense ministry said three of its soldiers were killed. 

And a representative of Nagorno-Karabakh's separatist leadership said four of its military personnel had died.  The region is a crossroads for strategically-important oil and gas pipelines. A return to war could also drag in the big regional powers, Russia and Turkey. Moscow has a defense alliance with Armenia, while Ankara backs Azerbaijan.