The militants escaped under the cover of darkness.
Haokip was not present and in his office at the time of the incident after the tree plantation function at the catchment area of Singda dam in Churachandpur district,they said adding that his family members were, however, inside the house.
Talking to reporters at his office last evening, Haokip said police bomb experts later removed the grenade and wondered why security personnel could not detect movement of ultras, moving with explosives in high security Imphal area.
The minister said the pin of the grenade was found outside the fencing of his wall though the grenade fell in his bedroom.
Haokip said some Kuki militants had earlier threatened him with dire consequences if their monetary demand was not met.
Meanwhile, newly-formed Kuki National Front-N (KNF-N) Publicity Secretary Gou Vaiphei told local media that they were responsible for the grenade attack at the minister's house.
He said the outfit had demanded money from the minister for 'some party fund' but he refused to pay the amount.
The minister said he had applied for government accommodation in the high-security Babupara ministerial complex but was yet to get it.