A visit to India by US senator John McCain on Wednesday was overshadowed by a row over reports that the National Security Agency was authorised to spy on Prime Minister Narendra Modi's Bharatiya Janata Party (BJP) in 2010.
McCain cancelled a news conference due to be held outside India's foreign ministry after India summoned a senior US diplomat to the ministry over the spying report.
US and Indian officials gave differing explanations for the cancellation, but said it was not linked to the row.
India sought an assurance that any such surveillance would not recur. "We expect a response to be provided to us, and if these (reports) are true, an assurance that this will not happen in the future," a senior Indian official told Reuters.
Indo-US relations have been delicate for months, following a major spat over the treatment of Indian diplomat, Devyani Khobragade, who was arrested in New York in December, an incident that was widely blamed for the resignation of the US ambassador to India. The Obama government has sought to revive ties since Modi's election in May, with an eye to ramping up bilateral trade and defence deals with India.
Modi was for years denied a visa for travel to the United States following religious riots while he was chief minister of Gujarat. He has responded positively to the US advances and shown no resentment publicly.
McCain, whose Arizona constituency is host to some of Boeing and Raytheon's most important defence businesses, told the Senate last week that Washington should seek to help India's economic and military development.
US secretary of state John Kerry is also expected to visit India soon.
The United States does not currently have an ambassador in New Delhi and its most senior diplomat is the charge d'affaires.
BJP was among a handful of political organisations a US court allowed the intelligence agency to spy on, according to a 2010 classified document leaked by former security contractor Edward Snowden and published by the Washington Post.
