Fighting that erupted Friday in the contested region of Nagorno-Karabakh was described as the heaviest hostilities since the 1994 ceasefire between Armenia and Azerbaijan. David Mdzinarishvili/Reuters

UPDATE: 6:35 p.m. EDT -- In a statement issued Saturday, U.S. Secretary of State John Kerry condemned the ceasefire violations in the Nagorno-Karabakh region, saying the United States urges both Armenia and Azerbaijan to "show restraint" and "avoid further escalation."

Russian President Vladimir Putin also called for an immediate end to the hostilities, Agence France-Presse reported.

Original story:

A decades-old conflict between Armenia and Azerbaijan flared up again late Friday, with dozens of soldiers reported dead on each side after a skirmish in the contested region of Nagorno-Karabakh.

Eighteen Armenian soldiers died in the fighting, Armenian President Serzh Sarkisian said in a statement Saturday, calling the conflict the “largest-scale hostilities" since the two nations reached a ceasefire in 1994 after a six-year war left more than 30,000 dead.

A statement by Azerbaijan’s state media agency reported that 12 Azeri soldiers died in the fight. Armenian forces also shot down an Azeri helicopter and destroyed a tank in the battle. Two civilians, an Azeri and an Armenian, were also reported killed.

Each side placed the blame on the other for the outbreak of hostilities, which followed several weeks of rising tensions and recriminations in an area that regularly sees minor exchanges of small arms fire. Armenian Prime Minister Hovik Abrahamyan accused rival Azerbaijan of committing an “unprecedented act” of aggression.

In a statement Thursday, President Ilham Aliyev of Azerbaijan said the tensions “must be resolved based on United Nations Security Council resolutions and the principle of territorial integrity.”

Smoke rises after clashes between Armenian and Azeri forces in the Nagorno-Karabakh region, in this still image taken from video provided by the Nagorno-Karabakh region Defense Ministry, April 2, 2016. Nagorno-Karabakh Military Handout/Reuters

The conflict dates to the early 1990s, when ethnic Armenian separatists took control of the Nagorno-Karabakh region, which lies within Azerbaijan’s borders, with the backing of Armenian military forces. After the 1994 truce, the internationally unrecognized enclave has remained a semi-autonomous patroled by the Nagorno-Karabakh Defense Ministry.

The clash between the two former Soviet states also drew quick action from Moscow. Russian Defense Minister Sergei Shoigu contacted his counterparts in both feuding nations, Interfax reported.

European diplomats implored the two sides to seek an end to the recent flare-up, which the Armenian government said was the first in more than two decades to involve heavy weaponry.

“We strongly condemn the use of force and regret the senseless loss of life, including civilians,” co-chairs for the Organization for Security and Cooperation in Europe’s Minsk Group said in a statement emailed Saturday. “There is no alternative to a peaceful negotiated solution of the conflict and that war is not an option.”