Former Prime Minister Manmohan Singh is stable and under observation at All India Institute of Medical Sciences in Delhi after suffering reaction to a new medication and developing fever, hospital sources said on Monday.

The 87-year-old Congress leader was admitted to the hospital on Sunday evening after he complained of uneasiness. Sources said Singh had developed a reaction to a new medication and further investigation is being carried on him to rule out other causes of fever.

"Dr Manmohan Singh was admitted for observation and investigation after he developed a febrile reaction to a new medication," they said. "He is being investigated to rule out other causes of fever and is being provided care as needed. He is stable and under care of a team of doctors at the Cardiothoracic Centre of AIIMS.”

"All his parameters are fine. He is under observation at the AIIMS," a source close to him said.

Singh, a senior leader of the opposition Congress, is currently a Member of Rajya Sabha from Rajasthan. He was the prime minister between 2004 and 2014.

In 2009, Singh underwent a successful coronary bypass surgery at the AIIMS.