1. A computer-implemented method for key phrase detection comprising: updating, at a current time instance, a start state based rejection model having a single state and a key phrase model having a plurality of states and associated with a predetermined key phrase based on scores of sub-phonetic units representative of received audio input, wherein said updating comprises: providing a transition of a score from a particular state of the plurality of states of the key phrase model to a next state of the plurality of states of the key phrase model and to the single state of the rejection model; and generating a rejection likelihood score corresponding to the single state of the start state based rejection model and a key phrase likelihood score corresponding to the key phrase model; and determining whether the received audio input is associated with the predetermined key phrase based on the rejection likelihood score and the key phrase likelihood score.