Chairman of Nationalist Socialist Council of Nagaland (NSCN-K) SS Khaplang died on Friday at Takka in Kachin state of Myanmar at the age of 77.
Official sources said that the leader of the Naga insurgent group, who had been ailing for quite sometime, died following a cardiac arrest.
Khaplang was a Hemi Naga from Myanmar and was based mostly in that country where the outfit maintains several camps.
Since 1980's, the outfit has engaged in subversive activities, including attacks on security forces, extortion and looting.
Born in April 1940 in Waktham village, east of Myanmar's Pangsau Pass, Khaplang joined
the Naga nationalist movement as early as 1964 and was one of the key people in the formation of the NSCN.
However, Khaplang split and formed his own group NSCN(K) in 1988.
He formed the group following differences with the NSCN (IM) leaders Isak Chishi Swu and Thuingaleng Muivah. Swu died in June 2016 in a Delhi hospital.
Khaplang also entered into a ceasefire with the central government in 1997 but abrogated it on 28 March 2015.
Meanwhile, Naga Peoples' Movement for Human Rights (NPMHR) Secretary General Neingulo Krome said the news of Khaplang's sudden demise has come as a "rude shock".