US Secretary of State Condoleezza Rice has urged Congress to ratify a deal to give India civil nuclear technology.
Rice says the deal will increase US influence in India
She told a senate committee the deal was good for the US, India and also the international community.
Ms Rice dismissed critics' fears that it would undermine efforts to prevent the spread of nuclear weapons
The deal, which in effect marks a reversal of decades of US foreign policy, was struck by President Bush when he visited Delhi last month.
The US had restricted nuclear co-operation with India - which has not signed the 1972 Non-Proliferation Treaty (NPT) - since it first tested a nuclear weapon in 1974.
Question of Iran
Under the terms of the deal, energy-hungry India will get access to US civil nuclear technology.
In return, India will classify 14 of its 22 nuclear facilities as being for civilian use, and thus open to full international inspection.
The US Congress must agree to amend existing legislation for the deal to go ahead.
NUCLEAR POWER IN INDIA
India has 14 reactors in commercial operation and nine under construction
Nuclear power supplies about 3% of India's electricity
By 2050, nuclear power is expected to provide 25% of the country's electricity
India has limited coal and uranium reserves
Its huge thorium reserves - about 25% of the world's total - are expected to fuel its nuclear power programme long-term
Source: Uranium Information Center
Giving testimony to the Senate Foreign Relations Committee, Ms Rice tried to assuage some senators' concerns that the deal could impact on efforts to limit the spread of nuclear weapons.
In a sharp exchange, Democratic Senator Barbara Boxer raised questions about India's military relationship with Iran, highlighting reports that Iranian navy ships had recently visited an Indian port.
"There have been and probably will be Iranian port calls in a number of countries in a number of countries in the world," Ms Rice said.
Ms Boxer replied: "No, no, this isn't port calls, this is training of their military. Did you make this part of a deal, yes, or not, because the reason I'm asking is I think some of us would like to make it condition."
Ms Rice said there had been Iranian ship port calls in India. But she said: "The assertion we understand that they train Iranian sailors, is not right."
The BBC's Jonathan Beale in Washington says Ms Rice acknowledged the US had concerns about India's relationship with Iran but she still hopes to persuade a sceptical Congress to back the nuclear deal.
Ms Rice also defended the deal as ending India's isolation from international nuclear standards, saying past non-proliferation policies had not worked and that India was unlikely ever to sign up to the NPT.
Ms Rice rejected critics' claims that the deal could boost India's nuclear arsenal.
India is looking at nuclear power to meet its energy needs
"Civil nuclear co-operation with India will not lead to an arms race in South Asia," she said.
Ms Rice stressed the importance of the "deepening strategic partnership" between India and the US.
Obstructing or altering the terms of the deal could redouble "all the hostility and suspicion of the past", she warned Congress, recalling Cold War tensions.
She said the deal would improve energy security, by reducing India's reliance on fuel from Iran, and help the environment by cutting its carbon emissions.
The agreement has been welcomed by the International Atomic Energy Agency (IAEA), Britain, France and Russia, Ms Rice added.
However, many arms control experts are unhappy with the agreement and argue that Washington should have struck a harder bargain, the BBC's diplomatic correspondent Jonathan Marcus says.
The analysts fear assistance to India's civil programme could free-up additional radioactive material for bomb-making purposes, he says.