India says no dates have been fixed with the US for the final talks on a historic nuclear power deal.
Top US diplomat Nicholas Burns was due to travel to India in the second half of May to conclude the deal.
The US said talks earlier this month made "extensive progress", but some reports suggest the deal is in trouble.
The deal would let energy-hungry India import US civilian nuclear fuel, even though it has not signed the Non-Proliferation Treaty (NPT).
The talks have been slowed by differences over India's right to test weapons and reprocess spent fuel.
Critics of the deal say it could boost India's nuclear arsenal. They say it sends the wrong message to countries like Iran, whose nuclear ambitions Washington opposes.
The two countries also have serious differences over India's close relations with Iran.
India has insisted that the final agreement must not bind it to supporting US policy on Iran or prevent it from developing fissile material.
No prediction
India's External Affairs Ministry spokesman Navtej Sarna was asked on Thursday when the US's Undersecretary of State, Nicholas Burns, would next visit Delhi.
"No particular date has yet been finalised," he replied.
"We are in the process of exchanging suggestions and examining them. We will let you know when a visit is scheduled."
US officials deny that Mr Burns has cancelled his trip, because no date for it has yet been arranged.
A State Department spokesman told Reuters news agency that Mr Burns would go to Delhi "when we are ready to seal the deal".
He added, "We're not at that point yet. Nobody I've talked to offered a prediction as to when that would be."
The nuclear fuel deal was struck in July 2005 but some issues have remained stumbling blocks, despite President George Bush and Prime Minister Manmohan Singh appearing to have overcome the obstacles in March, 2006.
India and the US were once on opposite sides of the Cold War fence, but are now close allies.
But India's government has faced criticism at home that the deal will compromise its nuclear independence.
One crucial sticking point is over a clause saying the US would withdraw fuel and equipment if India breached its unilateral moratorium on nuclear tests.
India's military says a future nuclear test by Pakistan or China could compel it to follow suit.
Another key area of difference is over reprocessing. India wants complete freedom to process all of its spent fuel, while the US argues that material it provides must not be used for military purposes.
Under the agreement, India will get access to US civil nuclear technology and fuel, in return for opening its civilian nuclear facilities to inspection.
But its nuclear weapons sites will remain off-limits.
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