NEW DELHI (AFP) - India could lose out on a historic nuclear deal with the United States if it does not vote against Iran at a key meeting of the UN nuclear watchdog agency, the US ambassador to India warned.

Ambassador David Mulford also warned that Washington was not convinced by India"s statements on separation of its civilian and military nuclear programmes, a key pre-condition for the bilateral deal.

If India decides not to back an International Atomic Energy Agency (IAEA) resolution against Iran, "the effect on members of the US Congress with regard to (India-US) civil nuclear initiative will be devastating," Mulford told the Press Trust of India (PTI) news agency.

"I think the Congress will simply stop considering the matter. I think the initiative will die in the Congress not because the US administration would want it to," he said.

"This should be part of the calculations India will have to keep in mind," Mulford said.

"India will have to make a determination on what its national interests are. That is an issue firmly in the hands of the Indian government to decide," the US diplomat said.

Mulford said India will also need to convince the 44-member Nuclear Suppliers" Group (NSG), which, in addition to the US Congress, has to endorse the bilateral accord.

"It is not just the United States. (It"s) the NSG, which says, "Wait a minute, if we are going to make this very special one-time change, unique change for India in the nuclear field and they don"t stand up on this (Iran) issue, why should we make the change"," he said.

Mulford said the "observation" has been conveyed to officials in New Delhi.

"We have made it known to them that we would very much like India"s support because India has arrived on the world stage and is a very, very important player in the world.

"And if it opposes Iran having nuclear weapons, we think they should record it in the vote," he said.

India, which carried out a string of nuclear tests in 1998 and declared itself to be a nuclear-armed state, bristled at Mulford"s comments, saying New Delhi would be guided by its own national interest.

"We have seen the remarks attributed to the US ambassador in India concerning a possible vote on the Iran nuclear issue at the IAEA," foreign ministry spokesman Navtej Sarna said.

"The position that India will take on this issue at the IAEA will be based on India"s own independent judgement and we categorically reject any attempt to link this to the proposed India-US agreement on civil nuclear energy cooperation, which stands on its own merits.

"With regard to negotiations on the proposed agreement, India will proceed on the basis of its own national interests, as acknowledged by the US ambassador himself," Sarna said.

Under the Indo-US nuclear deal, signed by US President George W. Bush and Indian Prime Minister Manmohan Singh last July, India would get advanced nuclear technology which it has been denied for 30 years for conducting nuclear tests and refusing to sign on to the nuclear Non-Proliferation Treaty (NPT).

Mulford labelled the agreement as a once-in-a-lifetime deal.

"India is a non-signatory to the NPT and has in the meantime developed nuclear weapons (and yet) it"s being given a chance to become a single exception in the system," he said.

Under the historic accord, India must also separate civilian and military nuclear programs in exchange for advanced civilian nuclear technology. It would place its civilian nuclear reactors under IAEA inspection.

Mulford said Washington felt the ideas put forth by India on nuclear programme separation had not met the "test of credibility".

The negotiations process needs to be completed before Bush"s visit to New Delhi in March, he said, cautioning that if not, the "historic opportunity" would be "much less practical."

The comments came less than a week after US Undersecretary of State Nicholas Burns spoke in New Delhi of unspecified "difficulties" in finalising the pact.