By Hasmik Mkrtchyan and Nailia Bagirova
BAKU/YEREVAN (Reuters) - The Organisation for Security and Cooperation in Europe (OSCE) urged Armenia and Azerbaijan to honor their 20-year-old truce on Thursday, after Azeri forces shot down a military helicopter from a disputed region controlled by ethnic Armenians.
The incident on Wednesday was the first of its kind since a 1994 ceasefire was agreed after a war between the two countries over Nagorno-Karabakh, a breakaway region of Azerbaijan.
The OSCE, which has led talks between Azerbaijan and Armenia since the war, said it should not lead to new violence.
"The OSCE Chairperson-in-Office and Swiss Foreign Minister, Didier Burkhalter ... appeals to the Presidents of Azerbaijan and Armenia to do their utmost to prevent such incidents and to honor the ceasefire regime," the OSCE said in a statement.
Footage from the Azeri side showed the helicopter explode in the air over the neutral zone between the two sides and officials said the three crew were believed to have been killed, but there was no confirmation of the deaths on Thursday.
Armenia promised "painful" consequences for Azerbaijan after it downed the helicopter, which Nagorno-Karabakh said had been carrying out training exercises.
Azerbaijan said the aircraft was Armenian and had been trying to attack Azeri positions near the mined and heavily guarded line of contact around Nagorno-Karabakh.
The Azeri defense ministry said an army officer had been honored for shooting down the aircraft.
Baku and Yerevan accused each other of violating the ceasefire and each said the other was responsible "for the escalation of the situation".
The incident highlights the risk of tensions over Nagorno-Karabakh triggering a wider conflict in the South Caucasus, through which oil and gas flow from the Caspian region to Europe.
About 30,000 people were killed in fighting between ethnic Azeris and Armenians which erupted in 1991 as the Soviet Union collapsed.
Mainly Muslim Azerbaijan and predominantly Christian Armenia regularly trade accusations of inciting violence along the Azeri-Armenian border and around Nagorno-Karabakh.
The region runs its own affairs with heavy military and financial backing from Armenia.
The OSCE's Minsk Group, co-chaired by France, Russia and the United States, has failed to find a permanent settlement to the conflict and no peace treaty has ever been signed.
(Additional reporting and writing by Margarita Antidze; Editing by Thomas Grove and Andrew Roche)