Armenia and Azerbaijan were locked in conflict over Nagorno-Karabakh for nearly four decades after the Soviet Union they were both members of collapsed.

Armenia and Azerbaijan have fought a series of wars since the late 1980s, when Nagorno-Karabakh, a region in Azerbaijan that had a mostly Armenian population at the time, broke away from Baku with Yerevan's support.

But hurdles remain. Azerbaijan also said Armenia’s constitution must “eliminate the claims against the sovereignty and territorial integrity of Azerbaijan,” as a prerequisite to allow for the signing of the peace treaty.