The prisoner is on ventilator and in deep coma, hospital sources said.
Sanaullah was airlifted from Jammu to the Post Graduate Institute of Medical Education and Research (PGIMER) on Friday evening.
52-year-old Sanaullah is being looked after by a team of doctors, including neurosurgeons, in the intensive care unit.
A team of Pakistani High Commission officials which visited Sanaullah, a resident of Sialkot in Pakistan, at PGIMER yesterday, is expected to meet the doctors again today to seek a fresh update on his condition.
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Upon his arrival here, PGIMER said Sanaullah was checked by neurosurgeons at the Jammu hospital, where he was first rushed to after the attack on him, "who detected head injuries with a large fracture. CT scan showed multiple contusions with severe brain edema (brain capillaries damaged due to head injury)."
Two prison officials, including Jail Superintendent Rajni Sehgal, had been suspended by the Jammu and Kashmir government in connection with the assault.