Congress party leader Sonia Gandhi has travelled abroad for routine medical checks six months after undergoing surgery in the United States, a spokesman for the party said on Tuesday.

Gandhi will return in four to five days, Janardan Dwivedi said in a statement.

The Congress party has never disclosed 65-year-old Gandhi's condition. Some media reports said she had received treatment at New York's Sloan-Kettering Cancer Center.

During her absence last year, she devolved part of her responsibilities to her son, Rahul, increasing speculation that he may soon take over from his mother as party leader and, eventually, take over as prime minister from Manmohan Singh.

Italian-born Sonia Gandhi stayed out of the political limelight for several months after her surgery last August, a time when her party was grappling with corruption scandals and an ill-fated attempt to implement landmark reforms in India's retail sector.

But she returned to the campaign trail towards the end of last year to help lead the party in state elections that began last month. These include Uttar Pradesh, a state of 200 million people on which Rahul has staked his political future.

The Gandhi family, descended from India's first prime minister, Jawaharlal Nehru, enjoys a status similar to royalty in the country of 1.2 billion. They are not related to independence hero Mahatma Gandhi, a close ally of Nehru.

Out of respect, normally clamorous 24-hour news stations have been almost silent on Sonia Gandhi's condition or what her absence meant last year for running the world's largest democracy.

India's main political parties also largely shied away from commenting on her condition and last year's absence.

Sonia was married to Rajiv Gandhi, Nehru's grandson and a former prime minister, who was killed by a suicide bomber in 1991 while campaigning for elections.

His mother, Indira Gandhi, was also prime minister when she was assassinated by her Sikh bodyguards in 1984.

Sonia Gandhi is seen as the main driver for massive welfare and back-to-work schemes for the rural poor, a counterbalance to Singh's more reform minded agenda.

Often seen as the power behind the throne, after driving Congress to election victory in 2009 but declining to become prime minister, critics say Gandhi has been an ineffective leader who failed to halt corruption on her watch.