Armenia said on Thursday that forces of neighbouring Azerbaijan had killed two of its soldiers at the border between the two countries involved in an unresolved conflict that killed about 30,000 people in the early 1990s.

A military conflict between ethnic Azeris and Armenians erupted in 1991 over Nagorno-Karabakh, a mountainous enclave within Azerbaijan with a majority Armenian population. Armenian-backed forces seized the area and seven surrounding Azeri districts.

Armenia's Defence Ministry spokesman said the latest incident occurred miles from Nagorno-Karabakh, where sporadic violence still flares along a ceasefire line negotiated in 1994.

Skirmishes around Nagorno-Karabakh and along the border between Armenia and Azerbaijan underline the risk of broader conflict in the South Caucasus, where vital oil and natural gas flow from the Caspian region to Europe.

Azerbaijan's Defence Ministry spokesman declined to comment on the Armenian accusation.

The incident comes a day after the Organisation for Security and Cooperation in Europe (OSCE) head Didier Burkhalter visited the region and urged Armenia and Azerbaijan to find a peaceful solution to the conflict.

An Azeri officer was killed a few days ago at another part of the border and the country's defence ministry accused Armenia of violating the ceasefire agreement.

Efforts to reach a permanent settlement of the Nagorno-Karabakh conflict have failed despite mediation led by France, Russia and the United States.

Oil-producing Azerbaijan, host to global majors including BP , Chevron and ExxonMobil, frequently threatens to take the mountain enclave back by force, and is spending heavily on its armed forces.

Armenia, an ally of Russia, says it would not stand by if Nagorno-Karabakh were attacked.