Fighting on the border between Armenia and Azerbaijan killed about 100 troops Tuesday as attacks on both sides fed fears of broader hostilities breaking out between the long time adversaries.

Armenia said at least 49 of its soldiers were killed; Azerbaijan said it lost 50.

The fighting erupted minutes after midnight with Azerbaijani forces unleashing an artillery barrage and drone attacks in many sections of Armenian territory, according to Armenia's Defence Ministry. It said shelling grew less intense during the day but Azerbaijani troops were trying to advance into Armenian territory.

Azerbaijan's Foreign Ministry said it was responding to a "large-scale provocation" by Armenia late Monday and early Tuesday. It said Armenian troops planted mines and fired on Azerbaijani military positions.

The two countries have been locked in a decades-old conflict over Nagorno-Karabakh, which is part of Azerbaijan but has been under the control of ethnic Armenian forces backed by Armenia since a separatist war there ended in 1994.

Azerbaijan reclaimed broad swaths of Nagorno-Karabakh in a six-week war in 2020 that killed more than 6,600 people and ended with a Russia-brokered peace deal. Moscow deployed about 2,000 troops to the region to serve as peacekeepers under the deal.

The Russian Foreign Ministry on Tuesday urged both parties "to refrain from further escalation and show restraint." Moscow has engaged in a delicate balancing act in seeking to maintain friendly ties with both ex-Soviet nations. It has strong economic and security ties with Armenia, which hosts a Russian military base, while also has been developing close cooperation with oil-rich Azerbaijan.

The international community also urged calm.

U.N. Secretary-General Antonio Guterres urged Armenia and Azerbaijan "to take immediate steps to deescalate tensions, exercise maximum restraint and resolve any outstanding issues through dialogue" and implement previous agreements, his spokesman said.

The U.N. Security Council scheduled closed consultations Wednesday on the renewed fighting.

Armenian Prime Minister Nikol Pashinyan called Russian President Vladimir Putin and later also had calls with French President Emmanuel Macron, European Council President Charles Michel and Iranian President Ebrahim Raisi. Russian Foreign Minister Sergey Lavrov spoke by phone with his Azerbaijani counterpart, Jeyhun Bayramov.

U.S. Secretary of State Antony Blinken spoke with both Pashinyan and Azerbaijan's president, Ilham Aliyev. The U.S. has a special envoy in the region, Blinken said, "and my hope is that we can move this from conflict back to the negotiating table and back to trying to build a peace."

Speaking in parliament early Tuesday, Pashinyan accused Azerbaijan of having had an uncompromising stance at recent European Union-brokered talks in Brussels.

Armenia said the Azerbaijani shelling Tuesday damaged civilian infrastructure and wounded an unspecified number of people.

On Facebook, Aliyev expressed condolences "to the families and relatives of our servicemen who died on September 13 while preventing large-scale provocations committed by the Armenian armed forces in the direction of the Kalbajar, Lachin, Dashkasan and Zangilan regions of Azerbaijan."

Turkey, an ally of Azerbaijan, also placed the blame for the violence on Armenia. Turkish President Recep Tayyip Erdogan expressed support for Aliyev and said in a statement that Turkey and Azerbaijan are "brotherly ... in all matters."

The governor of Gegharkunik province, one of the regions that came under Azerbaijani shelling, said there was a 40-minute lull in the fighting, apparently reflecting Moscow's attempt to negotiate a truce, before it later resumed. The governor, Karen Sarkisyan, said four Armenian troops in his region were killed and another 43 were wounded by the shelling.

The Armenian government said it would officially ask Russia for assistance under a friendship treaty between the countries, and also appeal to the United Nations and the Collective Security Treaty Organization, a Moscow-dominated security alliance of ex-Soviet nations.

Kremlin spokesman Dmitry Peskov refrained from comment on Armenia's request but added during a conference call with reporters that Putin was "taking every effort to help de-escalate tensions."