The negotiations which began on Tuesday had not been affected by differences between Russia, the United States and other world powers over Ukraine, senior Iranian negotiator Abbas Araqchi was quoted on Wednesday as saying by ISNA news agency.

"The issue was not raised in the negotiations but it is normal that a crisis with such scope would overshadow other issues," Araqchi said.

The Vienna gathering is the second in a series of meetings aimed at transforming a November interim deal into a lasting accord that resolves for good the decade-old standoff over Iran's nuclear drive.

Under November's interim deal with the United States, China, Russia, Britain, France and Germany, known as the P5+1, Tehran froze certain nuclear activities for six months in exchange for relief from punishing sanctions hitting its economy.

"It is too early to enter into negotiations for drafting a text for a final agreement," said Araqchi, adding that another round of talks was slated for April 7 and 8 "probably in Vienna".

Under November's agreement, Iran froze key parts of its nuclear program in return for minor sanctions relief and a promise of no new sanctions for six months.

Although it could be extended, the deal is currently due to expire on July 20.

A key issue still to be resolved is the future of the Arak heavy-water reactor, which Iran says will be used for research but which world powers say could provide Tehran with an alternative route to an atom bomb.

Araqchi reiterated that Iran would not shut down the reactor but said it could take measures to allay Western concerns over the nuclear facility.

"There is a room for discussion about the size and extent of the enrichment, considering Iran's practical needs," he said.

BA/BA