Favouring early firming up of an agreement to operationalise the civil nuclear deal, the US has said the "terms" of the pact would have to be "right" for it to get passed by the Congress.

Ahead of a meeting between Prime Minister Manmohan Singh [Images] and President George W Bush in Germany [Images] during which the matter will come up, Washington said there are differences on a "couple of issues" in the 123 Agreement aired by India and further discussions are needed to resolve these.

"These are some pretty tough negotiations," State Department spokesman Sean McCormack told a media briefing in Washington on Tuesday.

Referring to last week's talks between Foreign Secretary Shiv Shankar Menon and Under Secretary of State Nicholas Burns in Delhi, he said, "We made some progress... There are going to be follow-up conversations."

Pressed on the kind of progress made, McCormack said, "I think they (India) aired some of their differences over a couple of issues" but did not specify.

"We would like to get this deal done as quickly as we possibly can," the State Department spokesman said.

"If we can get the terms of the nuclear deal right, that it will move through (the Congress)," he said when asked if the deal will get Congress approval.

The year-long talks have failed to yield any breakthrough due to differences on certain issues, particularly whether India should have right to reprocess spent nuclear fuel.

India has been insisting on getting this right and is also pressing the US to assure continued supplies of fuel and cooperation, even if it were to conduct a nuclear test.