Ninety-eight-year-old Singh, the only IAF officer of the IAF to be promoted to five-star rank, equal to a field marshal in the Army, was admitted to the Army's Research and Referral hospital this morning after he suffered a cardiac arrest, the defence ministry said.
"We are all praying for the speedy recovery of Marshal of the Indian Air Force Arjan Singh. Doctors are doing their best," the prime minister said.
Sitharaman expressed the hope that Singh will recover at the earliest, adding his condition was critical.
An icon in the country's military history, Singh had led a fledgling IAF in the 1965 Indo-Pak when he was just a 44- year-old.
As Pakistan had launched its Operation Grand Slam with an armoured thrust targeted at the vital town of Akhnoor in Jammu and Kashmir, Singh led the Indian Air Force through the war with courage, determination and professional skill.
He inspired the IAF, despite the constraints imposed on a full-scale use of air force combat power.
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