Mr Swu, 85, was admitted to the Fortis Hospital in south Delhi on July 5, 2015.
Isak Chisi Swu, co-founder of the National Socialist Council of Nagaland-Isak-Muivah (NSCN-IM), is being treated in New Delhi for kidney ailments, doctors said today.
In Nagaland, mass prayers are being held for the early recovery of Mr Swu, who along with Thuingaleng Muivah heads the NSCN-IM. The two have engaged in a dialogue with New Delhi since 1997 when they signed a ceasefire.
Mr Swu, 85, was admitted to the Fortis Hospital in south Delhi on July 5 following "severe kidney ailments and urinary tract infection", a doctor told IANS.
"Swu underwent a surgery last week and is currently in the ICU," the doctor said.
Refusing to divulge further information, the doctor said Mr Swu would not be discharged until he recovers fully.
A source told IANS that Mr Swu was in Delhi for the last one month and was treated for similar problems earlier also.
"Swu's two sons, who stay abroad, are also at the hospital," the source said.
Mr Swu, currently the chairman of NSCN-IM, was one of the key members in the peace talks between the government and the Naga group.
In Nagaland, church groups, individuals and others have been holding prayers for the recovery of the NSCN leader, said Chuba Ozukum, president of the Naga Hoho, the apex body of Nagaland.
The Council of Nagalim Churches (CNC) has thanked the people of Nagaland for the mass prayers organised in churches, villages, towns and individual homes across the state.
The general secretary of the Council of Nagalim Churches, Seksim Kasar, thanked the doctors and nurses caring for Mr Swu.
NSCN-IM officials said their leader was diagnosed with urinary tract infection a few years ago.
"His condition deteriorated a few days back but it is improving now, which is a good sign for the entire Naga nation," said a senior NSCN leader.
Senior NSCN-IM leaders and leaders of social groups in Nagaland have visited Delhi.
Hailing from Chishilimi village in Zunheboto district, Swu and NSCN-IM general secretary Thuingaleng Muivah have been the main people engaged in a dialogue with Delhi since 1997.
Mr Swu and Mr Muivah joined the Naga National Council in the early 1960s.