Frequent shootings have been reported along their common border since the end of the 2020 war between Yerevan and Baku for control of the breakaway region of Nagorno-Karabakh.
Fresh clashes have broken out on the Armenia-Azerbaijan border, it's been claimed.
Armenia's defence ministry claimed Azerbaijani forces had started "intensive bombardment" of its positions shortly after midnight towards several towns such as Goris and Sotk.
Armenian Prime Minister Nikol Pashinyan has claimed that at least 105 soldiers have been killed.
The Azerbaijan foreign ministry later said that 50 of its soldiers were killed and that “definitive retaliatory measures were taken”.
Baku accused Armenia of "large-scale subversive acts" overnight, adding that Armenian fire had caused casualties in its ranks.
Pashinyan held separate talks overnight with Russian President Vladimir Putin, his French counterpart Emmanuel Macron and US diplomatic chief Antony Blinken.
Armenia and Azerbaijan, two rival ex-Soviet republics in the Caucasus, have clashed in two wars over the past three decades for control of the Nagorny Karabakh region, the last of which was in 2020. Both sides have accused each other of violating the ceasefire agreement and launching new shells on Wednesday.
Turkish Foreign Minister Mevlut Cavusoglu said on Tuesday he had discussed "Armenian provocations" with Baku and called on Armenia to focus on peace negotiations.
In a statement, Russia said the matter should be resolved "exclusively through political and diplomatic means" and called on both sides to cease hostilities amid "extreme concern".
The European Union's foreign policy chief Josep Borrell called for a halt to hostilities and "a return to the negotiating table".
The French Elysée Palace also said on Tuesday that it would bring the dispute to the United Nations Security Council.
UN Secretary-General Antonio Guterres has 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.
Six weeks of fighting in the autumn of 2020 left more than 6,500 people dead and resulted in a ceasefire brokered by Russia.
As part of the deal, Armenia ceded parts of the territory it had controlled for decades and Moscow deployed some 2,000 Russian peacekeepers to oversee the fragile truce.
Frequent shootings have been reported along their common border since the 2020 war.
Last week, Armenia accused Azerbaijan of killing one of its soldiers in border clashes. In August, Baku said it had lost a soldier, and the Nagorno-Karabakh army reported that two of its soldiers had been killed and more than a dozen wounded.
Another bloody war in the early 1990s saw thousands killed on both sides and hundreds of thousands of people displaced. That war ended with a truce in 1994, although there has been sporadic violence since as the dispute remains unresolved.