New Delhi/Kohima, June 28: Isak Chishi Swu, chairman of the most powerful rebel group in the Northeast, passed away in New Delhi today after a prolonged illness.

The 87-year-old NSCN (I-M) chairman was undergoing treatment for almost a year at a private hospital where he breathed his last this afternoon, Nagaland government spokesperson Kuolie Mere said.

Soon after his death, his body was shifted to the All India Institute of Medical Sciences (AIIMS) for embalming. It will be flown to Nagaland tomorrow after a memorial service at Nagaland House in the heart of the capital in the morning. The funeral is likely to be held in his native place on Thursday.

Swu hailed from Zunheboto district of Nagaland and belonged to Sumi community. He is survived by his wife Khuli and their six children.

Junior home minister Kiren Rijiju said the Naga leader's death had left a vacuum but that would not impact the final Naga peace accord. "It is sad news but there won't be any impact on the status of peace accord signed last year," Rijiju said. "Discussions are still going on with Naga groups and there are several contentious issues which need to be sorted out. This will take a little longer..." he added.

The NSCN (I-M) had signed a framework agreement with the Centre in August last year. It was described by Prime Minister Narendra Modi as a "historic" step to usher in peace in the state. The pact was signed by NSCN (I-M) general secretary Thuingaleng Muivah and government interlocutor R.N. Ravi in the presence of the Prime Minister, home minister Rajnath Singh and national security adviser Ajit Doval at the Prime Minister's residence, while Swu had signed it in the hospital he had been admitted to the previous month for multiple health problems.

The agreement was signed after over 80 rounds of negotiations between the NSCN (I-M) and the Centre, spanning 16 years. Official peace parleys begun the two parties had begun in 2000, after the outfit entered a truce with the Centre in 1997.

Swu had joined the Naga National Council (NNC), which had emerged as the voice of Naga freedom, in the 1950s. He served the NNC in various capacities till 1980, when he, Muivah and their NNC comrade-in-arms, S.S. Khaplang, split from the parent body in protest against the Shillong Accord, which was signed by the NNC with the Centre in 1975. The trio formed the NSCN.

The NSCN split again in 1988 into NSCN (I-M) and NSCN (Khaplang).

Over the years, the NSCN (I-M) has been accused of engaging in killings, extortion and other subversive activities. Its persistent demand for sovereignty led to military clampdown. The group tried to develop extensive links both within and outside India. It is also believed to have patronised smaller extremist outfits in the Northeast by giving warfare and intelligence training and providing logistics for waging war against India. It also opened up contacts with international organisations like the United Nations Human Rights Council in Geneva, Unrepresented Nations and Peoples Organisation (UNPO) at the Hague and the UN Working Group on Indigenous Populations.

Swu was among the first to have established links with China.

He wrote in his autobiography: "In 1968, I was sent to China as president's special envoy for diplomatic mission. We marched from Nagalim to China via Kachin land. It was a challenging and a tiring command. We had to go without food for nearly a month as we were chased everywhere by the Burmese army."

"Many Kachin soldiers laid down their lives for us while escorting our party. We will never forget those KIA who had sacrificed their lives for the Nagas....The Chinese authorities warmly welcomed us. I was honoured to talk to the then Premier of China on the situation in Nagalim. He assured us stating, 'When the right time comes we will be the first country to recognize your state, because you (the Nagas) have every right to be free'."

Speaking to reporters at AIIMS, Muivah said the Naga people had been hoping that Swu would be with them till the end of the solution. "Departure of the chairman is a shock," he said.

About the progress in peace talks, Muivah said, "Government of India is serious and we are also serious. Our commitment is there. We are hopeful." As for funeral arrangements, his said, "People all over Nagaland will be waiting for him. So we will have to respect their wish."

Naga Peoples Front MP Neiphiu Rio, who visited AIIMS to pay tribute to Swu, told The Telegraph that the vacuum created by his death could not be filled but the organisation he built was such that his legacy would be carried forward.

Prime Minister Narendra Modi condoled Swu's death in a series of tweets. "Mr. Isak Chisi Swu wished the best for the Naga people and aspired for peace. Mr. Swu will be remembered for his historical role in bringing out the Framework Agreement for Naga peace. My heartfelt condolences to the family and supporters of Mr. Isak Chisi Swu on his demise. May his soul rest in peace."

Congress president Sonia Gandhi hoped that Swu's legacy would evolve into that of lasting peace and reconciliation.