Agartala, July 12: Maximum alert has been sounded in Tripura to deter Pakistani Inter-Services Intelligence (IS) agents and insurgents being trained in Bangladesh from entering the state through the porous border, according to the director-general of police (DGP) K T D Singh. Singh said that Tripura police suspected the hand of ISI and Bangladesh outfit Harkat-ul-Zihad in using landmines for the first time in an ambush by militants at Takumbari in South Tripura on July 10. Six persons, including five Tripura State Rifles (TSR) jawans, were killed in the ambush. The DGP, who visited the spot after the incident, told newsmen here on Sunday night that a strong group of 20 militants had launched the ambush in a planned manner using four landmines. They also hurled eight to nine grenades. He said the identity of the insurgents involved in the ambush was not ascertained but they were suspected to belong to the banned outfit, All Tripura Tiger Force (ATTF).