Authorities in the Armenian region surrounded by Azerbaijan have said at least 84 fighters were killed in clashes.
At least 11 civilians have died, nine of them in Azerbaijan.
So far, Azerbaijan has not reported the deaths of soldiers.
The Armenian Ministry of Defense said Monday night that Azerbaijani forces had launched a large-scale attack.
Azerbaijan and Armenia have been fighting for Nagorno-Karabakh for a long time and the region was fought in the 1990s.
The UN Security Council is convening for an emergency meeting on Tuesday because of the situation in the region.