Artillery strikes on civilians in the Nagorno-Karabakh conflict could amount to war crimes, the UN human rights chief said on Monday, reiterating a call for Azerbaijan and Armenia to halt attacks on towns, schools and hospitals in the region.

Separately, Armenia's prime minister called for an investigation into the presence of "foreign mercenaries" in Nagorno-Karabakh after ethnic Armenian forces said they had captured two fighters from Syria. Azerbaijan denied the presence of foreign combatants.

Fierce battles continued along the front line of a conflict that has killed at least 1,000 people, and possibly many more.

UN High Commissioner for Human Rights Michelle Bachelet said indiscriminate attacks in populated areas in and around the conflict zone contravened international humanitarian law.

She said in a statement that repeated calls for both Armenia and Azerbaijan to avoid the loss of civilian life and damage to civilian infrastructure had gone unheeded.

"Instead, homes have been destroyed, streets reduced to rubble, and people forced to flee or seek safety in basements," she said. "Such attacks must stop and those responsible for carrying them out, or ordering them, must be held to account."

Just hours after agreeing in Geneva on October 30 to avoid the deliberate targeting of civilians, Azerbaijan and ethnic Armenian forces in Nagorno-Karabakh accused each other once more of shelling residential areas.

Citing data from both sides of the conflict, Bachelet said about 40,000 Azeris had been temporarily displaced by the latest fighting while some 90,000 ethnic Armenians had fled Nagorno-Karabakh and were currently in Armenia.

(With input from Reuters)