VIENNA (Reuters) - A U.S.-led push for India to join a club of countries controlling access to sensitive nuclear technology made some headway on Thursday as several opponents appeared more willing to work towards a compromise, but China remained defiant.

India's Prime Minister Narenda Modi arrives for the first plenary session of the Nuclear Security Summit in Washington April 1, 2016. REUTERS/Jonathan Ernst

The 48-nation Nuclear Suppliers Group aims to prevent the proliferation of nuclear weapons by restricting the sale of items that can be used to make those arms. It was set up in response to India’s first nuclear test in 1974.

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.

But China on Thursday maintained its position that the Non-Proliferation Treaty is central to the NSG, diplomats said.

The handful of other nations resisting India’s admission to the group, including South Africa, New Zealand and Turkey, softened their stance somewhat, opening the door to a process under which non-NPT states such as India might join, diplomats said.

“There’s movement, including towards a process, but we’d have to see what that process would look like,” one diplomat said after the closed-door talks on Thursday aimed at preparing for an annual NSG plenary meeting in Seoul later this month.

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

Pakistan joining would be unacceptable to many, given its track record. The father of its nuclear weapons program ran an illicit network for years that sold nuclear secrets to countries including North Korea and Iran.

“By bringing India on board, it’s a slap in the face of the entire non-proliferation regime,” a diplomatic source from a country resisting India’s bid said on condition of anonymity.

Washington has been pressuring hold-outs, and Thursday’s 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.

Most of the hold-outs argue that if India 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 now backs India’s membership bid. One Vienna-based diplomat said it had softened its stance but still opposed the idea of India joining under conditions that did not apply equally to all.