Vienna, Jun 9 (Reuters) : China is leading opposition to a push by the United States and other major powers for India to join the main club of countries controlling access to sensitive nuclear technology, diplomats said on Thursday as the group discussed India's membership bid.

Other countries opposing Indian membership of the Nuclear Suppliers Group (NSG) include New Zealand, Ireland, Turkey, South Africa and Austria, diplomats said.

The 48-nation NSG aims to prevent the proliferation of nuclear weapons by restricting the sale of items that can be used to make those arms.

India already enjoys most of the benefits of membership under a 2008 exemption to NSG rules granted to support its nuclear cooperation deal with Washington, even though India has developed atomic weapons and never signed the nuclear Non-Proliferation Treaty (NPT), the main global arms control pact.

Opponents argue that granting it membership would further undermine efforts to prevent proliferation. It would also infuriate India's rival Pakistan, which responded to India's membership bid with one of its own and has the backing of its close ally China.

"By bringing India on board, it's a slap in the face of the entire non-proliferation regime," a diplomatic source from one of a handful of countries resisting India's push said on condition of anonymity.

A decision on Indian membership is not expected before an NSG plenary meeting in Seoul on June 20, but diplomats said Washington had been pressuring hold-outs, and Thursday's closed-door meeting was a chance to see how strong opposition is.

U.S. Secretary of State John Kerry wrote to members asking them "not to block consensus on Indian admission to the NSG" in a letter seen by Reuters and dated Friday.

China, however, showed no sign of backing down from its opposition to India joining unless Pakistan becomes a member. That would be unacceptable to many, given Pakistan's track record -- the father of its nuclear weapons programme sold nuclear secrets to countries including North Korea and Iran.

"China, if anything, is hardening (its position)," another diplomat said.

Most of the hold-outs oppose the idea of admitting a non-NPT state such as India and argue that if it is to be admitted, it should be under criteria that apply equally to all states rather than under a "tailor-made" solution for a U.S. ally.

Mexico's president said on Wednesday his country supports India's membership bid, but one Vienna-based diplomat said it still opposed the idea of it joining under conditions that did not apply equally to all.