The comments made to Timothy Roemer last year were immediately criticised by the main opposition Hindu nationalist Bharatiya Janata Party (BJP), adding to political sparring that has deadlocked parliament and pushed policymaking into limbo.
Gandhi's comments, made in response to a question from Roemer on the Lashkar-i-Taiba (LeT), referred to religious tension created by more extreme BJP leaders, according to the cable released by WikiLeaks and published on Friday by Britain's Guardian newspaper.
Gandhi said there was evidence of some support for the LeT cable.
"However, Gandhi warned, the bigger threat may be the growth of radicalised Hindu groups, which create religious tensions and political confrontations with the Muslim community," Roemer wrote.
India has a history of communal tension between majority Hindus and minority Muslims, and critics say several political parties play on that tension to win votes. In 2002, about 2,500 people, most of them Muslims, were killed in riots in western Gujarat state, human rights groups estimated.
Radical Hindu groups, some with ties to the BJP or the BJP's more extreme sister organisations, have been linked to bomb attacks against Muslim targets.
The controversy adds to the woes of the ruling Congress party to which Gandhi belongs, which is already fighting to contain the damage from a series of setbacks including corruption scandals, high food prices and poor showings in state elections.
The BJP has threatened to block a February budget session of parliament if the government does not set up a parliamentary committee to investigate charges the country lost $39 billion in revenue due to corruption in the granting of telecoms licences.
Gandhi, son of powerful Congress chief Sonia Gandhi, has sparked the BJP's ire before. He once compared the opposition party's parent organisation to the banned Students Islamic Movement of India.
On Friday, the BJP said Gandhi's comments were adding grist to propaganda from militants and Pakistan.
"In a way he is seeking to justify the entire propaganda. That will impinge on our security and strategic concerns and the fight against terror. The BJP strongly condemns this statement which is irresponsible," BJP leader Ravi Shankar Prasad said.
"After this, the people will know that you, Rahul Gandhi, have a long way to go to understand this nation." Gandhi and Congress spokesmen were not available for comment.
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