AGARTALA, May 30 ? Four jawans of Tripura State Rifles (TSR) were killed and ten others injured in an ambush by insurgents of banned National Liberation Front of Tripura (NLFT) at Ghagracherra on the Assam-Agartala national highway in Dhalai district, police said today, reports PTI. The ultras ambushed a TSR vehicle when the jawans were returning to their camp at Gaydabari in North Tripura district after admitting a sick jawans at GB Hospital yesterday, the police said. The TSR jawans opened fire in retaliation when the insurgents showered bullets on the bus from an advantageous position on a hillock and the encounter lasted for about two hours. Meanwhile, a police convoy led by subdivisional police officer (SDPO) Timir Das arrived on the spot and the ultras fled from the scene leaving four jawans dead and injuring ten others.