Washington has expressed concern about the downing by Azerbaijani forces of a military helicopter from the self-proclaimed "self-defense forces" of Nagorno-Karabakh -- a breakaway region of Azerbaijan that is controlled by ethnic Armenians.
U.S. State Department spokeswoman Jen Psaki says the incident is "yet another reminder" of the need to reduce tensions and respect a cease-fire in the region.
Psaki told reporters in Washington on November 12 that there could be "no military solution to the conflict."
The Minsk Group of mediators from the Organization for Security and Cooperation in Europe (OSCE) -- which is co-chaired by France, Russia, and the United States -- said the incident shows the "urgency to intensify efforts to find a lasting settlement" to the decades-old conflict.
Nagorno-Karabakh: Why Are They Fighting?
Azerbaijan and Armenia have been at loggerheads for decades over the status of Nagorno-Karabakh, a mountainous region inhabited almost entirely by ethnic Armenians but which is located within Azerbaijan’s internationally recognized borders.
The Soviet government designated the territory an autonomous region within Soviet Azerbaijan in the 1920s. Under Moscow’s iron rule, violence between the Armenians and Azerbaijanis that predated their incorporation into the Soviet Union was kept largely in check.
But as the Soviet Union began to wobble in the late 1980s, simmering tensions boiled over into a six-year war after Nagorno-Karabakh sought to formally join Armenia. The region’s self-styled government declared unilateral independence in 1991, and an estimated 30,000 people died in the conflict before Russia brokered a cease-fire in 1994.
The territory occupies some 4,400 square kilometers of land, and Armenian forces control buffer zones surrounding the breakaway region.
Nagorno-Karabakh has maintained de facto autonomy since the cease-fire, while Azerbaijan maintains its claim to the region.
Internationally mediated negotiations by the Minsk Group co-chaired by France, Russia, and the United States and organized under the Organization for Security and Cooperation in Europe (OSCE) have failed to yield a resolution to the so-called frozen conflict.
Cross-border violence has remained constant in the two decades since the 1994 cease-fire but has not escalated into a full-scale war.
-- Carl Schreck, Luke Johnson
Azerbaijan’s Defense Ministry says Azerbaijani troops shot the helicopter down on November 12 as it "attempted to open fire" on them.
The ministry said: "According to preliminary information, the three servicemen of Armenian military forces on board were killed."
Later, Baku announced it was decorating soldier Ilkin Muradov -- who was credited with shooting down the helicopter.
David Babaian, a spokesman for the Nagorno-Karabakh forces, said the helicopter was on a training mission and denied it was involved in any combat operations.
Armenia's Defense Ministry spokesman Artsrun Hovannisyan called the incident an "unprecedented provocation that leads to an escalation in the situation."
Hovannisyan called Azerbaijani claims the helicopter attacked Azerbaijani troops "absurd."
He also said: “The consequences for the Azerbaijani side will be very painful, and all responsibility will rest with the Azeri political leadership."
It was the first time a military aircraft has been shot down in the Nagorno-Karabakh region in at least 20 years.
In Brussels, EU foreign policy chief Federica Mogherini said "it is essential that all sides show restraint and avoid any actions or statements which could escalate the situation."
Mogherini also called for an inquiry into the incident.
Azerbaijan and Armenia have been locked in a dispute for years over Nagorno-Karabakh.
Armenian-backed separatists seized the mainly Armenian-populated region from Azerbaijan during a war in the early 1990s that killed some 30,000 people.
Despite years of internationally brokered talks, the two sides have not yet signed a peace agreement, with Karabakh still internationally recognized as part of Azerbaijan.
Azerbaijani and Armenian forces regularly exchange fire across their frontier and along the Karabakh frontline.
An unprecedented spiral of violence this summer -- with more than 20 troops killed from both sides -- raised concerns of a new war.
Oil-rich Azerbaijan, which has a large defense budget, has said it could be a retrograde step for the region if diplomacy fails.