Troops from Azerbaijan and ethnic Armenian forces engaged in a fourth day of intense fighting around the disputed territory of Nagorno-Karabakh on September 30 that threatens to draw Russia and regional power Turkey into a wider conflict.
Dozens of people, including civilians, have been killed and scores injured as both sides use armed drones, tanks, helicopters, and multiple-rocket systems.
The violence is the worst in years, and possibly the worst since a 1994 cease-fire ended outright war between Armenia and Azerbaijan.
Both sides on September 30 confirmed heavy clashes along much of the line of contact that separates the ethnic Armenian forces that control Nagorno-Karabakh from Azerbaijan troops.
The fighting has threatened to draw in Russia, a member of a security alliance with Armenia, and NATO member Turkey, which said that it will back Azerbaijan with "every means available" in the conflict.
Deepening concerns, Russia's Foreign Ministry mentioned reports that fighters from Syria and Libya were being sent to join the fighting in Nagorno-Karabakh.
The ministry did not say who was sending what it called "illegal militia members" nor did it say on whose side they allegedly would be fighting on.
There have been unconfirmed reports that Turkey is sending fighters it backs in Syria to join the fighting on the Azerbaijani side.
"Intense fighting continues," Azerbaijan's Defense Ministry said in a statement on September 30, adding that military operations were under way "along the entire front line."
Armenian Defense Ministry spokesman Shushan Stepanyan said in a Facebook post that "artillery engagement continues along the entire front line in [Nagorno-Karabakh] at present."
Russia and Western nations have called for restraint, with the Kremlin urging all countries, especially Turkey, to "do everything to convince the opposing parties to cease fire and return to peacefully resolving the conflict by politico-diplomatic means."
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Russia's foreign minister, Sergei Lavrov, offered to host foreign ministers from both Armenia and Azerbaijan for talks. It was unclear if the offer was being considered. Russia is among the largest supplier of weaponry to both Azerbaijan and Armenia. It also has a military base in Armenia.
The UN Security Council strongly condemned the use of force and backed a call from Secretary-General Antonio Guterres for an immediate halt in the fighting, a de-escalation of tensions, and return to meaningful negotiations.
In dueling interviews with Russia's state-run TV network, the leaders of the two nations, however, brushed off the suggestion of peace negotiations, accusing each other of obstructing the process.
Azerbaijan and Armenia have been locked in conflict over the mountain region since the waning years of the Soviet Union. They fought a war that ended in 1994 with an uneasy cease-fire and an estimated 30,000 killed.
Since then, the region has been under the control of ethnic Armenian forces. The region's 1991 declaration of independence has not been recognized, though Armenia, which is its sole outlet to the outside world, has hosted official representative offices of the region's administration.
Sporadic violence has broken out over the years, and internationally mediated negotiations with the involvement of the OSCE's so-called Minsk Group, co-chaired by France, Russia, and the United States, have failed to achieve a resolution.
It wasn't immediately clear what caused the long-simmering conflict to erupt anew on September 27.
Both sides have accused each other of attacking civilian targets and firing into their territory outside of the Nagorno-Karabakh area.
Separatist fighters have reported at least 84 servicemen killed over the past four days. Azerbaijan has reported the death of 12 civilians on its side but released no figures on its military casualties.
The European Court of Human Rights called on Armenia and Azerbaijan to avoid any military action that could endanger civilians and warned military forces against using torture or inhumane treatment against captives.
The call followed a request from the Armenian government asking the court to weigh in on the current fighting.
Human Rights Watch urged all forces to "respect the absolute ban against targeting civilians or carrying out attacks that indiscriminately harm civilians."
On September 29, Armenia claimed that one of its warplanes was shot down by a fighter jet from Turkey, killing the pilot -- in what would be a major escalation.
Both Azerbaijan and Ankara denied this, but the Armenian government published on September 30 photos it claims show the wreckage of an Su-25 warplane shot down by a Turkish fighter jet.
Azerbaijan asserted that two Armenian Su-25 jets had crashed and were destroyed.
Armenia's prime minister, Nikol Pashinian, has accused Turkey of sending Turkish advisors and that officers were directly involved in the fighting -- an allegation that has not been independently confirmed.
Turkey's foreign minister did little to allay fears of a broadening conflict, saying that Ankara will provide support if Azerbaijan asks for it.
Pashinian, meanwhile, spoke by phone with Russian President Vladimir Putin on September 29 and said that, at this point, he was not considering asking for help under a post-Soviet treaty, called the Collective Security Treaty Organization, though he reserved the right to do so.
During a visit to Latvia on September 30, French President Emmanuel Macron criticized Ankara for issuing "warlike messages." However, he said he had no proof of regional players such as Turkey being involved so far.
After a phone call on September 30, Putin and Macron called for an immediate cease-fire and discussed steps the OSCE's Minsk Group could take to de-escalate the conflict.
Macron is expected to talk with U.S. President Donald Trump the next day.