UNITED NATIONS, Sept. 28 (Xinhua) -- United Nations Secretary-General Antonio Guterres on Monday called on Azerbaijani and Armenian leaders to take immediate steps to establish a ceasefire.

"The secretary-general spoke today with H.E. Mr. Ilham Aliyev, President of the Republic of Azerbaijan and with H.E. Mr. Nikol Pashinyan, Prime Minister of Armenia," according to a press note issued by the UN chief's spokesman Stephane Dujarric.

"The secretary-general expressed grave concern over the ongoing clashes along the line of contact in the Nagorno-Karabakh conflict zone. He urged both leaders to take immediate steps to establish a ceasefire and resume negotiations in order to prevent instability in the region," it said.

"The secretary-general reiterated his full support for the important role of the OSCE (the Organization for Security and Cooperation in Europe) Minsk Group co-chairs, stressing the need for the sides to work with them to return to negotiations without delay or pre-conditions," it added.

Forces of Armenia and Azerbaijan reportedly continued fighting over the Nagorno-Karabakh conflict zone for a second day on Monday, despite calls from the international community for the hostilities to end.

The UN chief on Sunday expressed his great concern over the latest Nagorno-Karabakh conflict, calling on both sides to immediately stop fighting.

Armenia and Azerbaijan have been at loggerheads over the mountainous region of Nagorno-Karabakh since 1988. Peace talks have been held since 1994 when a ceasefire was reached, but there have been occasional minor clashes along the borders.