A 46-nation export control group has acted to bar states that shun a global anti-nuclear weapons pact from obtaining technology which can be used to make atomic bombs, diplomats and experts say.

Last week's decision by the Nuclear Suppliers Group to tighten guidelines for transfers of sensitive uranium enrichment and reprocessing technology may irritate nuclear-armed India, after Washington helped it win a waiver from NSG rules in 2008.

The NSG -- which includes the United States, Russia, China, European Union countries and some others -- tries to ensure that nuclear exports are not diverted for military purposes.

India already has enrichment and reprocessing capabilities and does not need more advanced equipment of this type for its nuclear industry, arms proliferation expert Daryl Kimball said.

But the more stringent guidelines may still be seen as a blow for the Asian power.

"The Indians are going to cry foul. They want to be able to say that they are under no nuclear technology trade restrictions and that they are a responsible nuclear power," said Kimball, director of the Washington-based Arms Control Association.

"It is about prestige. It is not about any technical need."

India has so far not commented about the revised NSG guidelines, which have yet to be made public.

To import nuclear goods, all nations except the five officially recognised atomic weapons states must usually place nuclear sites under safeguards of the International Atomic Energy Agency, the U.N. nuclear watchdog, NSG guidelines say.

But after Washington sealed a nuclear supply accord with India, it won a unique exemption in 2008 following contentious negotiations. India gained access to technology and fuel while it was allowed to continue its nuclear weapons programme.

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 U.S. firms.

The NSG's revision of its voluntary guidelines -- under discussion for years and adopted at a June 23-24 meeting in the Dutch town of Noordwijk -- does not apply to trade in reactors or in the uranium needed to fuel them, experts say.

But an added condition stipulates that only parties to the nuclear Non-Proliferation Treaty (NPT) can get uranium enrichment or spent fuel reprocessing equipment and technology.

FLAW FIXED

This would prevent all NPT outsiders -- India, Pakistan, Israel and North Korea -- from importing such items, which can have both civilian and military applications.

India and Pakistan -- which have fought three wars and have tested nuclear arms -- have both refused to sign the 189-nation NPT, which is a cornerstone of global disarmament efforts.

"The new (NSG) guidelines include language saying transfers of enrichment and reprocessing technologies should be limited to NPT states and India doesn't qualify," proliferation expert Mark Hibbs of the Carnegie Endowment for International Peace said.

"India has been trying to get that particular item out of the new guidelines and they failed," Hibbs said. "It limits their access to sensitive technology."

Kimball said the revised wording helped "fix a key flaw" in India's waiver, ensuring that sensitive atomic items were not transferred to the country and used in its military programme.

One diplomat who attended the NSG meeting said the move should not be a surprise for India as it had been well-flagged already at the time of the 2008 exemption.

He also suggested there would be little practical impact as India had not been seeking -- and no NSG state had intended to sell it -- this kind of technology even before the amendment.

"The key issue for India is image. The NSG exemption gave India the appearance of being drawn into the mainstream. This change in the guidelines affects that view of itself," he said.

But David Albright, president of the Institute for Science and International Security think-tank, said India often tried to obtain dual-use goods for its gas centrifuge work from abroad.

"If the new NSG guidelines impose new conditions or new attention on these dual-use purchases, India may encounter more problems getting the necessary goods for its unsafeguarded nuclear programmes," Albright said.

Washington last year announced support for Indian membership of the NSG and a State Department spokeswoman on Friday said there had been progress in "bringing India closer" to the group.

But Hibbs said the latest NSG decision underscoring the NPT requirement "basically pushes back the deadline" for any decision on Indian membership.

Kimball criticised the idea of opening the door to India joining, saying it would further harden Pakistan's resolve to produce more fissile material for nuclear weapons.