US Undersecretary of State Nicholas Burns has been holding talks with the Indian foreign ministry on a landmark nuclear deal between the two countries.
India must declare its civilian and military nuclear facilities separately
Mr Burns and Indian Foreign Secretary Shyam Saran completed the first of two days of talks in Delhi on Thursday.
The two countries are trying agree on the implementation of the controversial deal which was signed last July.
It paves the way for US support of India's civilian nuclear programme. But there are still differences to resolve.
But both countries are keen to solve these issues soon, especially before the US President George Bush's forthcoming India visit.
Under the terms of the deal, India must declare its civilian and military nuclear facilities separately.
Civilian nuclear facilities would become eligible to use US fuel and technology - and subject to international inspection.
India wants to rid itself of US sanctions imposed in 1998
But military facilities would not.
Making that distinction between the two is also not easy.
Many of India's big research facilities are thought to straddle both fields.
As they struggle to sort out the details, both governments also face opposition at home.
In India, some opposition figures complain this enforced separation will undermine India's flexibility in the future.
And to go ahead with the deal, the Bush administration needs the approval of Congress.
Sceptics in the US accuse the administration of double standards, of forgiving India's nuclear transgressions because Delhi is an ally while lambasting Iran and North Korea.
Others say US support will allow India to dedicate its existing nuclear fuel stocks to its military programmes.
Despite their critics, both governments have strong motives for pressing ahead.
India is keen to emerge from the sanctions regime imposed on it after its nuclear tests in 1998.
It sees itself as a responsible member of the international community - despite its refusal to sign up to the main non-proliferation treaty (NPT).
The US meanwhile is eager to endorse India as a close ally and a strategic counterweight to an increasingly powerful China.
Washington is also anxious to stop India from going ahead with a high-profile energy deal with Iran.
India says it needs the Iranian gas pipeline to feed its expanding energy needs.
The US hopes to use the promise of civilian nuclear co-operation in its battle to persuade Delhi to say no to Iran and walk away from the deal.