Human rights groups called for an immediate halt to the use of banned weapons after confirming the use of cluster munitions by Armenia in an attack on the Azeri city of Barda.
The worst fighting in the South Caucasus for more than 25 years has raised fears of a wider war. It also poses a threat to pipelines carrying oil and gas from Azerbaijan to world markets.
Nagorno-Karabakh is internationally recognised as part of Azerbaijan, but is under the decades-long occupation of the Armenian side. About 30,000 people were killed in a 1991-94 war in the region.
Three ceasefires have failed to halt the latest fighting, the most recent brokered in Washington last Sunday by U.S. Secretary of State Mike Pompeo.
Sporadic fighting continued on Friday. Azerbaijan's defence ministry said military positions and settlements in the Aghdere, Khojavend and Gubadli regions had come under fire overnight.
Rights group Amnesty International and New York-based Human Rights Watch said they had independently confirmed the use of cluster munitions by Armenia's occupying forces in an attack on Barda on Wednesday. Azerbaijan has said 21 people were killed.
Both groups called for an immediate halt to the use of banned weapons.