New York [US], September 28 (ANI): Amid the ongoing clashes between Azerbaijan and Armenia, United Nations Secretary-General Antonio Guterres on Sunday (local time) said that he is concerned with the fresh "resumption of hostilities" between the two countries.

"The Secretary-General strongly calls on the sides to immediately stop fighting, de-escalate tensions and return to meaningful negotiations without delay," Guterres's spokesman said in a statement.

At least 23 people were killed and over 100 sustained injuries on Sunday (local time) as heavy clashes broke out between Armenian and Azerbaijani forces over the disputed separatist region of Nagorno-Karabakh, Al Jazeera reported.

Meanwhile, US State Department Spokesperson Morgan Ortagus condemned the clashes between the two countries and said: "The US is alarmed by reports of military action along the Line of Contact between Armenia and Azerbaijan. We extend our condolences to the families of those killed and injured. We remain committed to helping the sides achieve a peaceful settlement."According to Al Jazeera, Armenia has declared martial law and ordered its military to mobilise after a major "flare-up in violence" with Azerbaijan broke out over the disputed region.

Sputnik quoted presidential aide Hikmet Hajiyev as saying that Azerbaijan President Ilham Aliyev has approved the introduction of martial law in several regions of the country with a curfew set to go into effect on September 28.

Meanwhile, both have blamed each other for the escalation that led to casualties. Armenia accused the neighbour Azerbaijan "of hurting civilian settlements" in Nagorno-Karabakh -- which is recognised as a part of Azerbaijan internationally but controlled by Armenian forces.

Azerbaijan launched what it described as a "counteroffensive," while the self-proclaimed Republic of Artsakh accused Azerbaijani forces of opening fire against civilians and civilian infrastructure in its capital of Stepanakert. (ANI)