Heavy fighting has broken out between Armenian and Azerbaijani forces along the front lines of the separatist Nagorno-Karabakh region, the two sides said, disputing who had violated a fragile ceasefire again.
Both sides report numerous casualties, accusing each other on Saturday of violating a ceasefire, a sign that the two-decade-old conflict which has left some 30,000 people dead is far from a peaceful resolution.
The Armenian Defence Ministry said in a statement that Armenian anti-aircraft forces downed an Azerbaijani helicopter after Azerbaijan “made attempts to get deep into the defence lines of the Nagorno-Karabakh Army of Defence and capture tactical positions,” using tanks, artillery and aviation.
"Azerbaijani authorities bear all responsibility for the unprecedentedly supercharged situation," the statement added.
The breakaway region’s military reiterated the claims of the Armenian Defence Ministry, but the report was denied by Azerbaijan.
The Azerbaijani Defence Ministry said the fighting began when Armenian forces fired mortars and large-caliber artillery shells across the front line. Ministry spokesman Vagif Dargyakhly told The Associated Press that more than 120 shots were fired, some of which hit civilian residential areas.
There were no immediate figures on casualties.
Nagorno-Karabakh, which lies inside Azerbaijan but is controlled by ethnic Armenians, has run its own affairs with heavy military and financial backing from Armenia since a separatist war which erupted in 1991 ended in three years later.
Despite years of negotiations under the Organization for Security and Cooperation in Europe (OSCE) Minsk Group - consisting of Russia, France and the US - little progress in resolving the dispute and violence has sporadically broken out since, with a similar incident taking place last month.
The conflict between the two countries started with Armenia's territorial claims against Azerbaijan in 1988 during the decline of the Soviet Union, in which both Azerbaijan and Armenia were constituent states.
Nagorno-Karabakh had held a referendum in December 1991 in order to create an independent state which meant a declaration of separation from the Republic of Azerbaijan. The referendum, which was boycotted by most of the local Azerbaijanis, was claimed to be accepted by the majority of the region.
However, it was stated that the referendum was illegal because according to the Soviet constitution, only its 15 republics could declare independence from the union and Nagorno-Karabakh was not a republic.
War quickly broke out and Armenian armed forces occupied 20 percent of Azerbaijan, including the Nagorno-Karabakh region and seven surrounding districts.
The most dramatic massacre in the conflict occurred in February 1992 when Armenian and some CIS (Commonwealth of Independent States) forces killed at least 161 ethnic Azerbaijani civilians in the Khojaly Massacre.
Today the sides are separated by a demilitarised buffer zone, with both claiming frequent violations by the other.
US, Russia call for calm
Russian President Vladimir Putin called for an immediate end to fighting along the Nagorno-Karabakh frontline after Saturday’s clashes.
"President Putin calls on the parties in the conflict to observe an immediate ceasefire and exercise restraint in order to prevent further casualties," Kremlin spokesman Dmitry Peskov told Russian news agencies.
US Vice President Joe Biden also "expressed concern about continued violence, called for dialogue, and emphasised the importance of a comprehensive settlement for the long-term stability, security, and prosperity of the region," the White House said in a statement after he met with Armenian President Serzh Sargsyan on the margins of the Nuclear Security Summit on Thursday.
On Wednesday, US Secretary of State John Kerry also called for "an ultimate resolution" of the conflict during talks with Azerbaijani President Ilham Aliyev at the State Department.
"We want to see an ultimate resolution of the frozen conflict of Nagorno-Karabakh that needs to be a negotiated settlement and something that has to be worked on over time," Kerry said during a brief photo opportunity with Aliyev.
Azerbaijan has long demanded Armenia returns its territory which it says is illegally occupied according to international law.
Armenia has not yet applied the UN Security Council's four resolutions on the liberation of the Nagorno-Karabakh and the surrounding regions.
Russia's military and political support has so far kept Armenia defiant against Azerbaijan, despite the fact that Azerbaijan’s military capacity has developed rapidly through the use of its substantial oil and gas revenues.
Russia has economic as well as strategic reasons for supporting Armenia, as many gas pipelines bringing Russian gas to the region run through Armenia.
Armenia is the only South Caucasian member of Russia’s Collective Security Treaty Organization (CSTO) which is regarded as Moscow’s new Warsaw Pact. Yerevan’s membership in the CSTO enables Russia to deploy its 102nd military base in Gymru near the Turkish border.