A senior U.S. official said here on Monday the upcoming nuclear talks between Iran and six major states could be difficult and uncertain, noting his country welcomes the open discussion of Iranian heavy water reactor.

Speaking on condition of anonymity before Tuesday's talks, the official said the negotiations are expected to be "complicated, difficult and lengthy."

"As President (Barack) Obama has said, it's probably we won't get an agreement as we wish," the official noted.

Diplomats in Vienna also told Xinhua that since there are too many important issues for all sides to resolve, the talks would be very tough and hardly make a comprehensive deal.

This round of nuclear talks in the Austrian capital are aimed at hammering out a comprehensive resolution to the decade-old Iranian nuclear issue.

In November, Iran and the P5+1 group, which consists of the United States, Russia, China, France, Britain and Germany, reached an interim deal in Geneva on the Islamic republic's nuclear program.

Under the deal, Iran suspended its 20-percent-uranium enrichment and started the process of diluting and oxidizing its 196 kg stockpile of 20-percent enriched uranium in exchange for a partial relief of the sanctions imposed on its energy and financial sectors.

On Iran's heavy water reactor in Arak, the U.S. official said his country welcomes the discussion of possible modifications of it.

"We are pleased to see Iranian head of atomic energy agency say that they were open to discussions of whether modifications would be viable," the official said.

Iranian energy chief Ali Akbar Salehi said earlier this month that Tehran is willing to address Western concerns about the Arak reactor by turning it into a light water reactor.

The West suspects the heavy water reactor in Arak might produce weapon-grade plutonium. Experts here told Xinhua if it could be turned into a light water reactor, it would be technically impossible for Iran to produce enough plutonium for a nuclear bomb.

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