'We want the government to withdraw all security forces from Garo Hills and suspend the combing operation against us or else we will bomb several areas in Garo Hills,' Kakam D. Shira, a GNLA rebel leader, said in a statement.
'There is no question of withdrawing the troops or suspending our combing operation against them. Instead, we will vigorously intensify our operations,' said S.K. Jain, intelligence chief of Meghalaya Police.
An alert has been sounded across the state, he added.
The GNLA's threat came after personnel of the Combat Battalion for Resolute Action commandos and Border Security Force were deployed in the Garo Hills region.
Shira said the outfit had decided to go ahead with the three-day dawn-to-dusk shutdown from Sep 12-14 in protest against the ongoing combing operations.
The Garo Hills region is being used as a safe haven by various militant groups.