VIENNA (Reuters) - The United States and India have begun talks to flesh out rules for Indian reprocessing of spent US atomic fuel, a crucial part of a nuclear cooperation pact critics fear will erode non-proliferation standards. Talks got under way in Vienna after the sides agreed during a New Delhi visit by Secretary of State Hillary Clinton on two sites for US firms to build nuclear reactors under the pact, which lifts a 34-year nuclear ban on atomic trade with India. The high-level negotiations were held on Tuesday and Wednesday under a cloak of secrecy, diplomats said. Both sides declined comment. It was not known when talks would resume. Washington and New Delhi have allowed up to a year to settle on ways to ensure spent US fuel is reprocessed in India for electricity at designated civilian reactors subject to UN inspections, not turned into plutonium for nuclear weapons. The worlds largest democracy is, like Pakistan and Israel, outside the Non-Proliferation Treaty. Finalized last year, the civil nuclear accord grants India access to nuclear equipment and materials from the United States and other major supplier powers, ending a long embargo imposed following New Delhis 1974 nuclear bomb test. Backers say the pact will help to meet soaring energy demand in India, cut fossil fuel emissions linked to climate change, open up a multibillion-dollar nuclear market and forge a US-Indian strategic partnership drawing India into the centre of efforts to prevent the further spread of nuclear weapons. Disarmament advocates say the accord, hatched by former US president George W Bush, undercuts the NPT by rewarding a rare non-member and sending the wrong message to other nations, like Iran, alleged to have violated the treaty. They fear Indias civilian and military atomic sectors are not adequately separated and that importing nuclear material could let India use more of its limited indigenous supplies for its bomb programme, and drive Pakistan into another arms race. The US has made an exception of India while, with Iran in mind, seeking to tighten restrictions on exports of nuclear fuel enrichment and reprocessing technology by the 45-nation Nuclear Suppliers Group. G-8 industrialised powers endorsed that policy at a summit in Italy two weeks ago, prompting Clinton to reassure New Delhi during her visit that the US-India deal would not be affected. We have just completed a civil nuclear deal with India. If it is done within the appropriate channels and carefully safeguarded (against diversions into bomb making), as it is in the case of India, then that is appropriate, said Clinton. But we are very much opposed to unauthorised, inappropriate transfers that unfortunately can take place by certain countries or non-state actors doing so, she told reporters on Monday. The cooperation pact requires India to subject 14 of its 22 current or planned nuclear reactors to International Atomic Energy Agency inspections by 2014. India has yet to specify which reactors will come under monitoring.