The agitators, led by Pynshkemlang Nongshlong, rejected a written assurance from the Meghalaya government saying it had taken up the matter with the Centre and sought its "immediate intervention" and engaging the states for an early solution to the vexed problem.
"We want a joint assurance from the chief ministers of both Meghalaya and Assam," he told reporters at Langpih village near here.
The fast is being held at Langpih near the place where four youths were shot dead by Assam police during a clash in 2010.
The assurance signed by F Kharlyngdoh, secretary in the political department of Meghalaya, also said a special development package had been initiated for the areas bordering Assam, which is expected to be sanctioned expeditiously.
The construction of a road to Langpih would also be expedited, it said.
Kharlyngdoh said the state government had "re-strategised" its approach for a favourable settlement of this issue.
"Obstruction by Assam officials to construction activities such as extension of individual dwelling units and community infrastructure would also be taken up," he said, adding that the entry of new settlers to Langpih area since 2010 in violation of the chief ministers' agreement would also be discussed.