Azerbaijan's defence ministry said 16 its soldiers had been killed over the last two days.
The death toll from both sides since the clashes in the Caucasus erupted on Friday is at least 64, according to an AFP estimate based on official reports.
"Armenia continued firing at Azerbaijani army positions and civilian targets using large-calibre machineguns and 120-millimetre mortars," the ministry said in a statement as clashes entered a fourth day.
The rebel defence ministry in Karabakh said that "Azerbaijan continued its aggression throughout the night."
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Russia and the West have hurriedly called for an end to the fighting.
Mediators from Russia, the United States and France were to meet in Vienna on Tuesday under the auspices of the Organisation for Security and Cooperation in Europe (OSCE).
Separatists backed by Yerevan seized control of mountainous Nagorny Karabakh, a majority ethnic Armenian region lying inside Azerbaijan, in an early 1990s war that claimed some 30,000 lives.
The sides have never signed a peace deal despite the 1994 ceasefire and sporadic violence on the line of contact regularly claims lives of soldiers on both sides, though the latest outbreak represents a serious escalation.