Appropriate treatment was being administered to him under the supervision of a team of doctors, it added.
Earlier, the hospital had said 93-year-old Vajpayee, who had not been keeping well for long, was admitted for routine check-ups and investigations and that his condition was stable.
A team of doctors, under the supervision of AIIMS Director Randeep Guleria, was attending to the nonagenarian leader, the hospital said.
A host of political party leaders, including Prime Minister Narendra Modi, visited the BJP patriarch at the hospital.
Modi met the doctors and enquired about Vajpayee. He also met his family members, according to an official statement. The prime minister spent nearly 50 minutes at the hospital, the statement added.
BJP veteran L K Advani, Home Minister Rajnath Singh, party chief Amit Shah, Health Minister J P Nadda, Environment Minister Harsh Vardhan and Congress president Rahul Gandhi were among those who paid a visit to the ailing leader.
The BJP said in a statement that Shah had a detailed discussion with the doctors about Vajpayee's treatment and also met the former prime minister's family members at the hospital.
Union minister Vijay Goel told reporters that Vajpayee was being treated for an infection in the urinary tract and hoped that he would be discharged tomorrow.
Vajpayee was thrice elected the prime minister between 1996 and 1999 and is the only non-Congress prime minister to complete the full term of five years, from 1999 to 2004.
As his health deteriorated, he had slowly withdrawn himself from public life and was confined to his residence for years.