By Douglas Busvine

NEW DELHI (Reuters) - Diplomats have quietly launched a new push to induct India into a club of nuclear trading nations, but rather than increasing stability in South Asia, the move could escalate strains with rival Pakistan.

The chairman of the Nuclear Suppliers Group (NSG) visited New Delhi recently to meet Foreign Minister Sushma Swaraj as part of a diplomatic "outreach" that seeks to build a consensus to admit India at its annual meeting next June.

Membership of the 48-nation club would bring India into the nuclear fold 41 years after it tested its first nuclear bomb, and give the nation of 1.25 billion a vested interest in curbing the world's most dangerous regional arms race.

"It's a very delicate process, but I think there is less and less justification for the impasse," Rafael Grossi, the Argentinian ambassador to Vienna who heads the NSG, told Reuters in an interview.

Yet there are doubts. For one, India has not signed the Non-Proliferation Treaty, or NPT, which seeks to prevent the spread of nuclear weapons.

And Pakistan, an ally of China, also aspires to join the NSG. With a history as a proliferator, Pakistan's accession would be a tough sell.

Because the NSG operates by consensus, admitting India alone would mean it could then bar its western neighbor from the club, potentially pushing Pakistan further to the fringes.

Meanwhile, Pakistan has been testing missiles that can reach all of India, and very short-range missiles that it insists could be used only if Indian troops cross onto Pakistani soil.

A seat at the NSG would strengthen India's geopolitical clout and help it capitalize on nuclear trade and technology transfer opportunities, while also raising concern in Pakistan.

"India has a nuclear deal with the U.S., with France, it will soon have deals with Australia and Japan. So all this will of course complement its effort to get into NSG," said a senior Pakistani security official with knowledge of nuclear issues.

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"But people don't understand that India will use all this additional fuel (through civil nuclear deals) to make energy and have a lot more left over to use to make weapons.

"So at the end of it, the need for even more deterrence from our side will grow, not decrease."

UPPER HAND

Pakistan sees a nuclear lead as vital insurance against possible aggression by its larger neighbor, and it appears to be gaining the upper hand over India in the nuclear contest.

Analysts Toby Dalton and Michael Krepon estimate Pakistan is producing 20 nuclear warheads a year to India's five.

Yet defending that lead is a "losing proposition" that imposes huge costs on Pakistan's economy and strains its social fabric, they said.

In a report for the Carnegie and Stimson think tanks, Dalton and Krepon argued Pakistan should abandon its goal of "full-spectrum" deterrence against India and satisfy itself with "strategic" deterrence, or the ability to launch an effective counter-strike in the event of an attack.

India and Pakistan have fought three wars since independence and partition in 1947, two over Kashmir. Their disputed frontier is one of the world's most heavily militarized regions. Border clashes and incursions pose a constant risk of escalation.

The U.S. State Department declined to comment on specific discussions over Pakistan, but an official said Washington had not entered into talks on a civil nuclear pact with it. Nor was it seeking a waiver for Pakistan to trade with the NSG.

The United States was continuing to integrate India into the "global non-proliferation mainstream", this official also said, adding that Washington supported India's membership in the four multilateral export control regimes. One of those is the NSG.

COMFORT LEVEL

India's long road to nuclear legitimacy began with a bilateral deal with the United States in 2005 that, three years later, yielded an exemption allowing it to trade in sensitive nuclear technology with NSG nations.

New Delhi expressed its interest in 2010 in formally joining the nuclear club.

But India's lobbying has met with scepticism from European countries like Austria and Switzerland, who have questioned its refusal to sign the NPT and give up nuclear weapons.

Indian negotiators now detect a change of tone, and are focusing on winning over European skeptics. That, in turn, could bring round China, they calculate.

"We are optimistic; there is a desire within the NSG to bring this process to a conclusion sooner rather than later," one Indian diplomat told Reuters. "People are comfortable with India."

Despite two summit meetings between Prime Minister Narendra Modi and Chinese President Xi Jinping, Beijing has yet to signal its assent and may not agree, analysts caution.

Despite those concerns, India is upbeat: "France joined the NSG before ratifying the Non-Proliferation Treaty," said the Indian diplomat.

"It's not about arms controls. It's about export controls."

(Additional reporting by Mehreen Zahra Malik in Islamabad, Idrees Ali in Washington and Adam Rose in Beijing; Writing by Douglas Busvine; Editing by Mike Collett-White)