US Defence Secretary Robert Gates has repeated American warnings to India that time is running out for an oft-delayed nuclear accord.
Defence co-operation between India and the US is increasing
Objections from the Indian government's communist allies have delayed the deal.
Mr Gates was speaking at the end of a two-day visit to India which focused on US arms sales and the growing military relationship between the two countries.
Under the terms of the controversial deal, India would get access to US civilian nuclear technology and fuel.
In return, Delhi would open its civilian nuclear facilities to inspection - its nuclear weapons sites would remain off-limits.
But various obstacles have prevented the agreement from being fully implemented.
The government says nuclear power can solve India's energy shortages
It has been vigorously resisted by India's communist parties, who argue that it would give the US undue influence over India's foreign and nuclear policy.
Any agreement would also eventually need to be approved by the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), as well as by the 45-nation Nuclear Suppliers Group, which regulates global civilian nuclear trade.
"The clock is ticking in terms of how much time is available to get all the different aspects of an agreement implemented," Mr Gates said.
He said the US would respect India's internal politics but was hopeful that the government would push the deal through parliament.
"It serves the best interests of both countries, and I think it is an agreement that has positive global consequences."
Earlier this month, a US delegation warned India that it must finalise the deal by July if Congress in Washington is to ratify it before American presidential polls.
Mr Gates said that regardless of what happened to the agreement, American military relations with India were broad and would continue to move in a positive direction.
There have been isolated protests against the nuclear deal
US arms contractors are competing for weapons contracts in India, including a deal to supply 126 multi-role fighter aircraft, worth $10 to $12bn.
Mr Gates said he had not just come just to persuade the Indian government to buy American military equipment.
He said he was more interested in broader military cooperation - better relations with India he argued, would not be at the expense of other countries, especially China.
"This is not a zero sum game," he said.