Indian National Security Adviser JN Dixit has died of a heart attack in the country's capital, Delhi.
Dixit was a career diplomat
He was appointed to the post by Prime Minister Manmohan Singh, following the Congress party-led coalition's election victory in May last year.
The 68-year-old was one of India's most distinguished diplomats and a former foreign secretary.
Observers say that as security adviser he played a key role in the recent improvement in relations with Pakistan.
"The nation has lost a true patriot, a great diplomatic and a wise strategist. The loss is irreparable," said Indian Prime Minister Manmohan Singh.
Analysts paid tributes to Mr Dixit's skills as a diplomat and security advisor.
"He was a man who brought a sense of realism to India's national security policy," said C Raja Mohan, a professor of international relations at Jawaharlal University in Delhi.
One of India's top nuclear scientists Anil Kakodkar said Mr Dixit "had been a strong support" to India's nuclear programme.
Mr Dixit was born in the southern city of Madras, and spent nearly four decades in India's foreign service.
He retired more than a decade ago to write and teach before returning to politics, and recently joined the Congress party.