Shelling was reported Monday in the disputed region of Nagorno-Karabakh, despite a cease-fire that took effect early Sunday.
Armenia and Azerbaijan each accused the other of shelling their cities overnight, despite a truce backed by international mediators aimed at quelling the violence, which has left hundreds dead.
Azerbaijani officials accused Armenia of shelling at least four regions outside the conflict zone overnight. Nagorno-Karabakh officials, as well as Armenian state media, said that Azerbaijan targeted multiple villages and resumed shelling overnight.
Both sides accused the other of violating the cease-fire Sunday, as well.
The latest cease-fire is the second brokered by Russian officials since the conflict began last month. Russia's foreign minister, Sergey Lavrov, who acted as a mediator for the truce, called the continued fighting unacceptable Monday.
United Nations Secretary-General Antonio Guterres also spoke out against the hostilities Sunday as he condemned "all attacks on populated areas impacted by the Nagorno-Karabakh conflict."
"As he underscored again in his latest calls with the foreign ministers of Armenia and Azerbaijan, both sides have the obligation under international humanitarian law to take constant care to spare and protect civilians and civilian infrastructure in the conduct of military operations," a statement released by spokesperson Stephane Dujarric read.
The ongoing fighting between Azerbaijan and Armenia erupted Sept. 27, marking the biggest escalation of the decades-old conflict over the breakaway region of Nagorno-Karabakh since a 1994 cease-fire.
The predominantly ethnic Armenian territory declared its independence from Azerbaijan in 1991 during the collapse of the Soviet Union, sparking a war that claimed the lives of as many as 30,000 people before a 1994 cease-fire. However, that independence is not internationally recognized.