Washington has warned India a landmark deal giving it US nuclear technology may fall through if Delhi does not back a UN motion against Iran.
India wants to use nuclear power to meet its energy needs
The deal could "die in Congress" if India does not vote against Iran at a meeting of the UN nuclear watchdog, US Ambassador David Mulford said.
The US is pursuing action against Iran over its apparent nuclear ambitions.
India says it rejects any attempt to tie its stance on Iran to its deal with the US on acquiring nuclear know-how.
'Test of credibility'
Washington agreed last year to share advanced civilian nuclear technology with Delhi, lifting sanctions triggered by India's nuclear tests in 1998.
The deal was struck by President George W Bush and Prime Minister Manmohan Singh but must be approved by the US Congress in order to be implemented.
US Ambassador to India David Mulford said the US is keen to have India's support when UN atomic watchdog, the International Atomic Energy Agency, meets to discuss Iran.
"India has arrived on the world stage and is a very, very important player in the world," he told the Press Trust of India.
"And if it opposes Iran having nuclear weapons, we think they should record it in the vote."
India's failure to do so, he said, would have a "devastating" effect on US Congress members who have yet to approve the nuclear deal.
Mr Bush and Mr Singh agreed the nuclear deal in 2005
"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.
"This should be part of the calculations India will have to keep in mind."
Mr Mulford also said the US had doubts about Indian assurances on the clear separation of its civilian and military nuclear programmes - a key condition of the technology-sharing deal agreed last year.
Ideas set out by India did not meet the "test of credibility", Mr Mulford is quoted by the AFP news agency as saying.
Negotiations on the issue must be completed by the time President Bush visits the country in March, he said.
Iran sanctions threat
Responding to the US ambassador's remarks, Indian foreign ministry spokesman Navtej Sarna said his country's agreement with the US on "civil nuclear energy co-operation... stands on its own merits".
The stance Delhi adopts towards Iran before the UN nuclear watchdog would be determined by its "own independent judgement", Mr Sarna said.
The IAEA meets next week to discuss referring Iran to the Security Council for possible sanctions.
Several Western nations accuse Iran of seeking to build nuclear weapons. Tehran maintains its nuclear programme has a purely civilian purpose.