Just days before President George W Bush's [Images] visit to India, Foreign Secretary Shyam Saran met US Under Secretary of State Nicholas Burns on Thursday to fine-tune the implementation of the civilian nuclear agreement.

They began their third round of talks to iron out differences on the deal on which the US says 90 per cent of negotiations have been completed.

• Complete Coverage: The Indo-US nuclear tango

However, after the meeting both sides admitted that differences persisted still and more negotiations will be required. Burns said he was not sure whether the deal would be finalised by the time Bush undertakes the three-day trip here from March one.

"Both of us want to complete these negotations but there are still some remaining differences between us and those differences need to be worked out," he told reporters.

"We simply do not know whether we will have an agreement before President Bush's visit. We are trying our best, both sides," Burns said, adding that though he had a good conversation with Saran but there are still differences remaining.

Referring to Bush's address to Asia Society in Washington on Wednesday, Burns said the President had made it clear that the separation plan must be transparent and credible and so we are still working on these issues. Saran also admitted to NDTV that differences remained between the two sides and said talks would continue on Friday.

Saran and Burns met for the third round of talks here to iron out differences on the deal on which the US says 90 per cent of negotiations have been completed. Burns and Saran, who will hold another session of talks on Friday, also met Prime Minister Manmohan Singh [Images] to brief him about the negotiations.

The two sides had encountered difficulties over the issue of separation, with the US insisting that India put more reactors than it was ready to in the civilian side. These included the homegrown fast breeder reactors, a move opposed by New Delhi.

Both countries are now understood to have expressed readiness to show flexibility on their respective positions.

India, on its part, is believed to have agreed to put at least half of 22 nuclear reactors in the civilian side while the US is understood to have accepted to leave FBRs out of the loop for next seven years. Burns and Saran also gave final touches to Bush's itinerary and the agenda of the talks between the US President and the prime minister.

The US under secretary's meeting with minister of state for external affairs Anand Sharma was cancelled at the last minute. Burns, who along with US Ambassador David C Mulford, reached South Block office to meet Sharma had to return when told that the minister was busy somewhere, sources said.