Sources said there is a possibility that the man, identified as Ahmed Mohammed, is lying but officials are checking his details to know where he travelled in the past few months.
The man, identified as Ahmed Mohammed, is said to be from Faislabad in Pakistan’s Punjab province.
Sleuths of the Intelligence Bureau (IB) was interrogating the man since he turned himself in to officials at the Indira Gandhi International Airport where he arrived early on Friday.
Mohammed is likely to be quizzed by the counter-intelligence head of IB when he returns from Srinagar on Saturday.
He was carrying a Pakistani passport – bearing the number KF 088779 – which showed his date of birth as July 9, 1978, the sources added.
Sources said he arrived in Delhi on an Air India flight and was scheduled to leave for Nepal on Friday morning.
Mohammed purportedly told his interrogators that he works for the ISI but wants to work for India now, the sources added.
News reports quoted Mohammed as revealing that the ISI had kept his family captive to prevent him from quitting his job as an assassin.
Both India and Pakistan frequently announce arrest of each other’s spies but instances of such agents turning themselves in to an enemy country is rare.
The incident also came a few days after Pakistan said a military court has sentenced an Indian national, Kulbhushan Jadhav, to death for spying. India has denied the charges.