The State Department made the request, even though the answers are not classified, because officials fear that public disclosure would torpedo the deal, sources said. The agreement would give New Delhi access to US nuclear technology for the first time since it conducted a nuclear test in 1974, but Left parties remain skeptical and view it as a possible infringement on India’s sovereignty.

Rep. Tom Lantos (D-Calif.), the late chairman of the House Foreign Affairs Committee, agreed to the request in February, and the current chairman, Rep. Howard L. Berman (D-Calif.), has abided by that commitment, though Berman is not considered a strong supporter of the deal. A group of prominent nonproliferation experts has decried the “virtual ‘gag’ order,” but thus far, the answers have not leaked, in part because only a handful of congressional officials have been able to read them.

“The administration’s unwillingness to make their answers more widely available suggests they have something to hide from either US or Indian legislators,” said Daryl Kimball, director of the Arms Control Association .

President Bush’s agreement with India, considered a key part of the administration’s foreign policy legacy, is designed to solidify Washington’s relationship with a fast-emerging economic power. Bush and Prime Minister Manmohan Singh agreed to the pact in July 2005, but it has faced many hurdles. If Congress gives the deal final approval, India will be able to engage in civil nuclear trade with the United States, even though it has not signed the nuclear Non-Proliferation Treaty.

The nearly 50 questions posed by Congress are highly technical, but they were carefully crafted to get to the heart of the balancing act the administration has performed between adhering to the letter of US nonproliferation law and assuaging Indian concerns that it was not being treated like a true nuclear power.

Congress passed a law, known as the Hyde Act, to provisionally accept the agreement, but some lawmakers have raised concerns about whether the implementing agreement negotiated by the administration fudges critical details.

For instance, one of the questions pertains to whether the United States would terminate nuclear trade if India resumes nuclear testing. This is a sensitive point in India and is required under US law, but the answer is not entirely clear from the text of the US-India agreement.

Another series of questions addresses the commitment by the United States to supply India with a “reliable supply of fuel” for its reactors, including a pledge to take steps to “guard against the disruption of fuel supplies.” A series of questions asks whether these commitments are legally binding, whether the two governments agree on the definition of a fuel supply disruption and whether the commitments would be affected by a nuclear test.

At one point, the lawmakers question whether these commitments in the implementing agreement are consistent with the Hyde Act.

Given the pointed nature of the questions, sources said the State Department had little choice but to be candid with lawmakers about the answers, in ways that senior State Department officials had not been in public.

Lynne Weil, a spokeswoman for the committee, said the State Department provided a lot of information, but the committee has agreed not to disclose the answers because “some data might be considered diplomatically sensitive.” She said the nuclear deal still must come back to Congress for final approval, and, at that point, public hearings will be held and “the questions will come up again.”