The United States is engaging in diplomatic outreach after Azerbaijan launched "anti-terrorist activities" in the Nagorno-Karabakh region on Tuesday, US officials said, adding that the incident was particularly dangerous.
A senior US State Department official said US Secretary of State Antony Blinken was likely to get involved in the next 24 hours in the diplomatic engagement already under way on the tensions between Armenia and Azerbaijan.
Armenia's Prime Minister Nikol Pashinyan and Blinken discussed the situation and stated the need for de-escalation, Interfax reported, citing the Armenian government.
Azerbaijan launched military action in the Nagorno-Karabakh region, a step that could presage a new war in the volatile area but which Baku said was necessary to restore constitutional order and drive out Armenian military formations.
A second senior State Department official said the incident overnight was "particularly egregious and particularly dangerous, so we'll obviously be in touch with all sides."
Karabakh is internationally recognized as Azerbaijani territory but part of it is run by breakaway ethnic Armenian authorities who say the area is their ancestral homeland. It has been at the center of two wars - the latest in 2020 - since the 1991 fall of the Soviet Union.
This week, the International Committee of the Red Cross (ICRC) was able to make simultaneous aid deliveries via the Lachin corridor and a separate road linking Karabakh to the Azerbaijani city of Aghdam.
Despite that, tensions have risen sharply this month, with Armenia and Azerbaijan accusing each other of building up troops.
"It's concerning that this happened overnight, especially because we did see some progress yesterday with shipments moving through the Lachin corridor," the first official said.
Armenia and Azerbaijan have already fought two wars over Karabakh in the three decades since the Soviet Union collapsed. Both had been part of the Soviet Union.
Analysts say successive rounds of talks, mediated variously by the European Union, the United States and Russia, have brought the two sides closer to a permanent peace treaty than they have been for years, but a final settlement remains elusive.