AGARTALA / AIZAL, Jan 25 (TIWN / IANS) The central government has alerted the northeastern states over possible terror attacks ahead of the Republic Day on Tuesday, a police official said on Monday. "The home ministry has alerted the northeastern states to maintain strict vigil in the run up to the Republic Day," Tripura Police spokesman Uttam Bhowmik told IANS. The Border Security Force (BSF) has also been told to further strengthen vigilance along the India-Bangladesh border to prevent trans-border movement of militants," he said. Security forces have been searching airports, hotels, bus and railway terminals as well as major markets across the region for possible hidden bombs. Four northeastern states - Tripura, Meghalaya, Mizoram and Assam - share an 1,880-km border with Bangladesh. India and Bhutan share a 643-km unfenced border. Four states - Mizoram, Manipur, Nagaland and Arunachal Pradesh - also have a 1,640-km unfenced border with Myanmar. And Arunachal Pradesh has an 1,080-km unfenced border with China. Several separatist outfits have called for a 24-hour general strike from January 25 midnight and a boycott of the Republic Day celebrations. The Northeast Frontier Railway has taken up security measures regarding rail traffic in southern Assam and elsewhere in the northeast.An official of the Airports Authority of India said entry of people, except passengers, into airports across the northeast had been barred. The restrictions would continue till January 29. Terrorist activities in Tripura reduced since a decade after Central Govt took positive steps to improve economy, employments and infrastructure development.