For over two decades, the Armenia-Azerbaijan conflict over the territory -- which no country recognises as independent -- has been a major source of tension in the strategic South Caucasus region wedged between Iran, Russia and Turkey (see background).

The dispute is rooted in a bloody war in the early 1990s, following the breakup of the Soviet Union. Yerevan-backed ethnic Armenian separatists seized control of Karabakh and several other regions of Azerbaijan during the conflict that left some 30,000 dead.

While Yerevan does not recognise Nagorno-Karabakh's independence claim, Armenia and rebel leaders pledged the elections would be free and fair, insisting that they were being held in line with international standards.

But oil-rich Azerbaijan, which has threatened to retake the region by force, condemned the election as illegal.

"The so-called 'elections' in Armenian-occupied Karabakh have no legal force, they contradict Azerbaijan's constitution and international law," Azerbaijan's foreign ministry spokesman Hikmat Hajiyev told AFP.

"Armenia stages a provocation called 'elections' which harms the ongoing negotiations."

'We will not accept results'

The European Union and the United States also weighed in this week on the Karabakh poll.

The EU's foreign policy chief, Federica Mogherini, said through a spokesperson on Friday that "the European Union does not recognise the constitutional and legal framework" within which the elections were to be held.

Jeff Rathke, the US State Department's acting spokesman, told journalists on Friday that "The United States does not recognise Nagorno-Karabakh as an independent sovereign state, and accordingly, we will not accept the results of the elections on 3 May."

Karabakh's ethnic-Azerbaijani community -- which before the war made up around 25% of the population -- was entirely driven out.

The enclave's 149,000-strong population is now mainly ethnic Armenian. Despite years of negotiations, the two sides have not signed a final peace deal, with Karabakh internationally recognised as part of Azerbaijan.

Baku, whose military spending exceeds Armenia's entire state budget, has threatened to take back the territories by force if negotiations fail.

Armenia, backed militarily by Russia, says it could crush any offensive.

Threatening a shaky truce, clashes between Azerbaijani and Armenian forces intensified again this year following an unprecedented spiral of violence in 2014.