Rahul Gandhi, widely tipped as a future Indian PM, believed Hindu radicals might be a greater threat than Islamist militants, diplomatic cables reveal.
According to Wikileaks, Mr Gandhi told a US envoy last year there was some support among Indian Muslims for militants such as Lashkar-e-Taiba.
But he told ambassador Timothy Roemer the greater threat could come from the growth of radical Hindu groups.
Opposition politicians were angered, suggesting Mr Gandhi has "a sick mind".
Mr Gandhi was at a lunch hosted by Indian Prime Minister Manmohan Singh for US Secretary of State Hillary Clinton in July 2009, Ambassador Roemer wrote in a cable a month later.
He recalls Mr Gandhi saying: "Although there was evidence of some support for Laskar-e-Taiba among certain elements in India's indigenous Muslim community, the bigger threat may be the growth of radicalised Hindu groups, which create religious tensions and political confrontations with the Muslim community."
In a statement released on Friday, Mr Gandhi - scion of India's Nehru-Gandhi political dynasty - stressed that "terrorism and communalism of all types" was a threat to India.
But opposition Hindu nationalist Bharatiya Janata Party spokesman Prakash Javdekar told AFP: "To call Hindu groups more dangerous than Lashkar-e-Taiba is the product of a sick mind."
Rahul Gandhi, who is secretary general of the Congress party, caused controversy in October when he compared a right-wing Hindu group with a banned Islamic group, saying both preached fundamentalist ideologies.
Other revelations relating to India in the latest Wikileaks cables include:
- US officials raised concern in June 2006 that terrorists could take advantage of lax security at Indian laboratories to steal "bacteria, parasites, viruses or toxins" and mount biological attacks across the globe
- During a chat in 2006 with California Governor Arnold Schwarzenegger's wife Maria Shriver, Rahul Gandhi's mother and Congress party president Sonia Gandhi appeared warm, admirable and informed, far from her reserved public self, and "despite her carefully erected Indian persona, her basic Italian personality is clearly evident in her mannerisms, speech and interests"
- A US briefing said in February this year that India was a "raucous democracy" and "a true partner" of America, and that their defence relationship was "progressing rapidly" with weapons sales to Delhi expected to reach $4bn in 2011
- The International Committee of the Red Cross sent evidence to US diplomats about widespread torture in 2002-04 by Indian security forces in Kashmir