He emphasised that those responsible for the "barbaric act" of beheading of Lance Naik Hemraj will have to be brought to book and hoped "Pakistan realises this".
In his public remarks on the January 8 incident on the LoC in Jammu and Kashmir in which two soldiers were killed and bodies mutilated, Singh said, "After this barbaric act there cannot be business as usual (with Pakistan)."
He was responding to reporters here at the Army Day reception hosted by Army chief Gen Bikram Singh when asked about the government's decision to put on visa-on-arrival scheme for Pakistan's elderly citizens.
When pointed out that Pakistan was in a denial mode on the cross-LoC attack, the Prime Minister said, "we will keep trying".
Asked about the options vis-a-vis Pakistan, he said these cannot be discussed in the open.
External affairs minister Salman Khurshid said India has taken a "very firm position" on the beheading issue with Pakistan as the act is "totally unacceptable and barbaric".
Referring to Monday's flag meeting between India and Pakistan on the LoC, he said, "we may not be happy with the outcome but the process is still on."