The US and India have failed to resolve differences over a proposed landmark deal on nuclear co-operation after three days of negotiations in Delhi.
India has pledged to open civilian nuclear sites to inspection
Indian Foreign Minister Shiv Shankar Menon said the two sides had made considerable progress, but that there were still gaps to be covered.
Under the deal, India would get access to US civilian nuclear technology if it opens its facilities to inspection.
Critics say the accord will encourage India to develop its nuclear arsenal.
They also say it sends the wrong message to countries like Iran, whose nuclear ambitions Washington opposes.
India has not signed the Nuclear Non-Proliferation Treaty (NPT).
After the intensive talks with US Undersecretary of State Nicholas Burns, Mr Menon said the two sides had made "considerable progress" towards completing the proposed deal.
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
"There are still issues where there are gaps," he said, but refused to give any specific details.
"We are optimistic that we will make the deal."
Mr Burns said the representatives had "useful discussions".
"While there has been good co-operation, more work remains to be done," he said.
"We look forward to a final agreement as it is indisputably in the interest of both governments."
'Infringement of sovereignty'
The BBC's Sanjoy Majumder in Delhi says the key sticking points were the issue of reprocessing spent nuclear fuel and carrying out more nuclear tests.
Washington is opposed to allowing India to undertake either, but the Indian government has said any restriction would be an infringement on its sovereignty.
India also wants the United States to guarantee its supply of nuclear fuel, our correspondent says.
The leaders of both countries are under considerable domestic pressure not to compromise, and with the US election approaching and the Indian government in the second half of its five-year term, time is running out for both administrations, our correspondent adds.
It is not clear when the next meeting will be held, although the leaders of both countries will have an opportunity to discuss the issue at next week's G8 summit.