The senators warn that the situation is getting critical

The civilian nuclear energy deal between the two countries has been delayed because of opposition from India's communist parties.

The parties provide key support to PM Manmohan Singh's minority government.

They argue that the accord would give the US undue influence over India's foreign and nuclear policy.

Under the terms of the deal, India would get access to US civil nuclear technology and fuel, in return for opening its civilian nuclear facilities to inspection.

But its nuclear weapons sites would remain off-limits.

The deal will 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.

'Difficult to ratify'

The delegation of three US senators arrived in Delhi on a day-long visit after monitoring Pakistan's elections.

"Time is of the essence," Joseph Biden, one of the senators, said, urging India to confirm its support for the nuclear deal.

The left-wing parties have been protesting against the nuclear deal

The visiting Americans say the deal is running out of time because of the tight legislative agenda for 2008 ahead of November's US presidential elections.

"If we don't have the deal back with us clearly prior to the month of July, it will be very difficult to ratify," Mr Biden said.

Whatever the merits of the deal, passing it through Congress after July would be harder because of "the mechanics on which our system functions", he said.

Mr Biden also said it was "highly unlikely" the next president would be able to authorise the same deal, if it did not reach Congress in time.

"It will be renegotiated," Mr Biden said.

"It is critical, if India want that deal, they move on relatively soon, within a matter of weeks,'' he said.

Mr Biden and senators John Kerry and Chuck Hagel met Prime Minister Singh on Wednesday to express their concerns over the issue.

India tested nuclear weapons in 1974 and 1998 and consequently is banned from buying fuel for atomic reactors and related equipment.

The deal has already been pronounced dead by some analysts because of India's domestic political rivalries.