An "optimistic" United States has denied that the Indo-US civilian nuclear deal was in "serious trouble" as it faces a crucial vote in the Senate and said it was hopeful that Capitol Hill will clear the deal.

Washington also said Secretary of State Condoleeza Rice was devoting "enormous time" by talking to lawmakers, who have raised some "hard questions", as the Bush Administration "still have work to do with the Hill" on the issue.

"We would like to say that we are optimistic about the deal going through in the Senate. But, again, we don't want to assume anything. We're not going to take anybody's vote for granted on the issue," State Department spokesman Sean McCormack said.

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Denying that the deal was in "serious trouble", McCormack said he had "not heard anyone using those terms in terms of the deal... our discussions with senators and Congressmen have been very positive on this."

"There are some people who have raised some hard questions about it and we try to answer those questions... we certainly are doing everything we can to encourage them to vote in the positive, working very closely with the leadership and people who are interested in this topic on the Hill, in terms of addressing their concerns, answering their questions," he said.

McCormack said Rice has been spending an enormous amount of time on the subject and the nuclear deal is among the top two of three issues that has merited her attention over the past few months.

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"She [Rice] has done quite a bit of work talking to members of Congress, both on the House of Representatives side, as well as the Senate side. A lot of meetings, a lot of phone calls since the deal was announced," McCormack said.

"She has done a lot of work with the Indians. There are still some issues that we're trying to work through with them. For example, the fissile material cutoff treaty negotiations in the Council on Disarmament in the UN, we just tabled a text there. It's certainly a text that we hope India can support. It's been an issue for some people."

McCormack said that "a variety of concerns" have been raised by law makers concerning India's participation in fissile material cutoff, it's participation in the Nuclear Suppliers Group and how the deal "does or does not limit their ability to produce more fissile materials that could go to a nuclear weapons programme".

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Asked what prevents him from saying "flat out" he was optimistic, he replied, "I'm not a vote-counter here. That's not my profession. It's just a way of saying that we're not going to take anybody's vote for granted. We're certainly not going to declare victory at this point.

"You don't want to assume anything. But we think we have a good case with this deal. We believe that we have a good case for the American people. It's a good case for the Indian people. And overall, it benefits the international non-proliferation regime and efforts.