Azerbaijani tanks shelled positions in the Nagorno-Karabakh region for the first time in 21 years, the Defense Ministry for the breakaway region said on December 9.
"For the first time since the cease-fire [in 1994], Azerbaijan has used tanks on the Karabakh front line," killing one soldier identified as Garik Avanesian, the ministry said.
"Some 1,500 shots were fired from tanks and grenade-launchers," it said.
Azerbaijan said Armenia had acted first, firing mortar rounds at settlements in Azerbaijan.
"The regime in Armenia bears responsibility for all of this," Azerbaijan's Defense Ministry said.
Azerbaijan's ministry warned it would launch retaliatory strikes on "enemy" army positions in Nagorno-Karabakh.
Armenia and Azerbaijan fought a bloody conflict over the disputed Nagorno-Karabakh territory in the 1990s before reaching a tenuous cease-fire.
The U.S. State Department condemned the violence and urged all sides to adhere to the cease-fire.
"The recent escalation of violence and the use of heavy weapons are unacceptable," spokesman John Kirby said.