Those for India joining say it is better if the country is inside than outside the NSG as it is already an advanced nuclear energy power and will in future become a significant exporter as well, one of the diplomats who attended the discussions said.
Those which are sceptical argue it could undermine the NPT, which is a cornerstone of global nuclear disarmament efforts.
"There are differences of opinion on allowing non-NPT members into the NSG," another diplomat said.
Mark Hibbs of the Carnegie Endowment think-tank said some "worried that India will use its voice to reverse the NSG's gears and loosen export controls, since India has not demonstrated a firm historical commitment" to its mission.
To receive civilian nuclear exports, nations that are not one of the five officially recognised atomic weapons states must usually place their nuclear activities under the safeguards of the International Atomic Energy Agency, NSG rules say.
When the United States sealed a nuclear supply deal with India in 2008 that China and others found questionable because Delhi is outside the NPT, Washington won an NSG waiver from that rule after contentious negotiations.
The landmark civilian nuclear cooperation agreement ended India's atomic isolation following its 1974 nuclear test and could mean billions of dollars in business for US firms.
India gained access to technology and fuel while it was allowed to continue its nuclear weapons programme.
Pakistan wants a similar civilian nuclear agreement with the United States to help meet its growing energy needs.
But Washington is reluctant, largely because a Pakistani nuclear scientist, Abdul Qadeer Khan, admitted in 2004 to transferring nuclear secrets to North Korea, Iran and Iraq.
Nuclear expert Daryl Kimball said India wanted to join the NSG because of prestige but that this would undercut the group's ability to ensure that New Delhi respects the non-proliferation commitments it made to win support for the 2008 exemption.
"Those commitments included no further nuclear weapons testing, compliance with site-specific safeguards, and support for a fissile material production moratorium," said Kimball, of the Washington-based Arms Control Association