US Senator John Kerry has said he backs a controversial nuclear accord with India "in principle".
India says it wants nuclear power to meet its energy needs
The landmark 2005 deal to grant India access to civilian nuclear technology must be ratified by the US Congress.
Critics of the accord, which hinges on India separating its military and civilian nuclear facilities, fear it could harm non-proliferation efforts.
Mr Kerry is a key player in the US Congress and observers say India needs his support to get the deal passed.
"In principle, I support this," Mr Kerry told reporters in Delhi, where he is on a three-day visit.
Mr Kerry, loser in the last US presidential election, is an influential Democrat member of the US Senate Foreign Relations Committee and an advocate of non-proliferation controls.
"It's a positive gain for India, the US and the international community," he said.
But he said the fine print of the deal would have to be studied before Congress voted on the issue.
"What Congress will or won't do is going to depend on what the four corners of the agreement finally say when it is arrived at."
Washington and Delhi have held several rounds of talks on the issue and a senior US State Department official, Nicholas Burns, is due to visit India later this month to hold more discussions.
Mr Kerry said that the nuclear deal would have large implications internationally.
Apart from being approved by Congress, he said it would need:
- to be approved by the 44-nation Nuclear Suppliers Group
- changes to be made to the Atomic Energy Advisory Board
- the adoption of the Fissile Technology Control Regime
During his visit, Mr Kerry held talks with Prime Minister Manmohan Singh and senior Indian officials.
Change in law
The accord was signed last July during a visit by Mr Singh to Washington.
It came as a boost for India, which has not signed the nuclear non-proliferation treaty, and therefore needs a change in US law.
Under the agreement, US companies will be allowed to build nuclear power plants in India, and also supply fuel for nuclear reactors.
The US imposed curbs on nuclear technology transfers to India in the wake of India's nuclear tests in 1998.
Delhi is keen on a deal on ways to share nuclear technology to help meet its growing energy needs.