Italian-born Gandhi is the widow of assassinated former prime minister Rajiv Gandhi [Reuters]
Sonia Gandhi, the head of India's ruling Congress Party, has undergone surgery in the United States for an undisclosed medical condition.
Without citing a source on Thursday, the Press Trust of India, the country's largest news agency, said: "It was learnt tonight that the Congress leader had undergone a successful surgery in a US hospital".
The agency said that it expected either Gandhi's family or her doctors to make a statement on Friday.
"On the advice of her doctors, she travelled abroad and she is likely to be away for two to three weeks," Janardhan Dwivedi, a Congress Party spokesman, told the AFP news agency.
India's Tehelka news magazine said on microblogging site Twitter that Gandhi, 64, was admitted on Thursday to New York's Memorial Sloan-Kettering Cancer Centre.
The absence of the figurehead of India's most prominent family dynasty may further hamper the Congress-led coalition government, which is already increasingly seen as rudderless amid a raft of corruption scandals and public fury over high inflation.
It could also accelerate the rise of her son, Rahul, one of a quartet of people appointed to take charge while she is away and considered to be India's prime minister-in-waiting.
"It is rather unusual. But it looks like a move to prepare Rahul Gandhi for his ultimate coronation," Amulya Ganguli, a political analyst told the Reuters news agency.
Apart from Rahul, the quartet comprises Dwivedi, Congress political secretary Ahmed Patel and Defence Minister AK Antony; all of them are seen as close advisers to Gandhi.
Behind the scenes
Italian-born Gandhi is the widow of assassinated former prime minister Rajiv Gandhi.
Her leftist leanings have proved a strong influence over the government of Manmohan Singh, the prime minister, and she has pushed pro-poor policies such as food subsidies and guaranteed employment for villagers.
She nominated Singh as prime minister in 2004, with many analysts considering her to be more powerful than him, governing India behind the scenes from her bungalow in the capital, New Delhi.
Her absence means she will miss the start of a crucial session of parliament which intends to introduce draft laws on a range of key areas, including fighting corruption, land acquisition and food security.