Indian Prime Minister Manmohan Singh underwent heart bypass surgery Saturday in an operation that raises questions about his political future.

The official Press Trust of India said the 76-year-old Singh, who had complained of chest pains earlier in the week and underwent a series of tests, went into surgery at about 9am local time (0330 GMT).

manmohan singhThe operation - taking place at the All India Institute of Medical Sciences (AIIMS) in New Delhi - was expected to last about six hours, with the prime minister likely to need close to a month of rest afterwards.

A hospital source said the doctors from AIIMS and Mumbai's Asian Heart Institute were "looking at the removal of two blockages. The rest of the regime will be decided when the surgery is on".

India is due to hold general elections before May, and the ruling Congress Party has so far insisted that the quietly spoken economist-politician Singh will be heading the party into the polls.

Largely been in good health

Congress had earlier this week dismissed concerns that Singh's health would interfere with its current election campaign, and has said the prime minister was "absolutely fine".

But there has been widespread speculation that party chief Sonia Gandhi has been lining up her son, Rahul Gandhi, heir to India's powerful Gandhi dynasty, as the country's next prime minister.

Singh underwent heart bypass surgery in Britain in 1990 and angioplasty in 2003.

He has largely been in good health since he was sworn in as prime minister in May 2004 but he recently underwent prostate surgery and has also had cataract treatment.

Government officials said that Foreign Minister Pranab Mukherjee will take charge of cabinet meetings during the prime minister's absence.

- AFP