The National Socialist Council of Nagaland (Isaac-Muivah) blamed the government on Monday for lack of progress in the long-running peace talks, saying New Delhi was not sincere about addressing their key demands.
The NSCN-IM, fighting for an independent homeland for the mainly Christian Naga people, held a new round of talks with government negotiators for three days in Amsterdam last week.
But no details were released.
"India is trying to test our patience by prolonging the peace process. Such an attitude of the Indian government will put at risk all peace initiatives in the region," Rh Raising, a senior NSCN-IM leader told Reuters.
The talks are mainly stuck over rebel demands to integrate all Naga dominated areas in the restive northeast region into a single state and their right to self-rule.
There was no comment from New Delhi over the outcome of the latest round of talks and officials said a statement could be expected after the negotiators brief senior government leaders.
More than 20,000 were killed in the conflict before the rebels and the government agreed to a truce in 1997. The ceasefire has held but the two sides have failed to find a political settlement to the revolt that began in 1947.
If the "casual attitude" of Indian officials continued, peace talks would prove futile and that would be expensive to both sides, Naga leader Raising said.
"We are sincere and committed in our efforts to find a peaceful settlement to the Indo-Naga problem, whereas India is committed to peace talks only in letter and not in spirit," Raising said.
Security analysts say peace with the Nagas is crucial to a broader peace in the northeast—seven states connected to the rest of India by a thin strip of land and home to dozens of insurgent groups.