Former Prime Minister Atal Bihari Vajpayee continued to be on ventilation on Saturday with his respiratory track infection remaining a cause of concern, doctors attending on him at the All India Institute of Medical Sciences said.

"Vajpayee's condition is stable. His heart is stable but his lungs require support and that is why we are putting him on ventilation," Dr Sampath Kumar, head of Cardiac Thoracic Vascular Sciences of AIIMS, said.

"His respiratory track infection is the cause of his problem. But his liver, kidney, blood pressure is normal. It will take some time before he recovers fully," he said.

Senior BJP leader L K Advani [Images] on Saturday spoke to doctors at AIIMS attending on former Prime Minister Atal Bihari Vajpayee, who is on ventilation due to respiratory tract infection, and enquired about his health.

"I have spoken to doctors attending on Atalji today morning and I was told that his condition was going to improve," Advani told reporters.

Congress president Sonia Gandhi [Images] today visited AIIMS to enquire about Vajpayee's health. Gandhi, who drove to the hospital today afternoon, spent some time with Vajpayee's foster daughter Namita.

She also spoke to doctors attending on the 84-year-old BJP veteran.

Kumar had on Friday said that the BJP leader's respiratory track infection had worsened. The 84-year-old leader was admitted to AIIMS on Tuesday with complaints of fever and chest congestion.

Doctors later found that he was suffering from pneumonia.

Vajpayee continues to be in the Intensive Care Unit, where his progress is being monitored by a team of doctors.