GENEVA, Oct 16 (Reuters) - Differences remain between Iran and the United States and other world powers over Tehran's nuclear ambitions, but the U.S. delegation has never had such intense talks with the Iranians as this week, a senior U.S. administration official said on Wednesday.
"I've been doing this now for about two years," the official said on condition of anonymity at the end of two days of talks between Iran and six world powers in Geneva. "And I have never had such intense, detailed, straightforward, candid conversations with the Iranian delegation before."
"Although there remain many differences in each area, and what sanctions relief might be appropriate, specific and candid discussions took place," the official added.
Earlier Western diplomats said Iran appears ready to scale back activities potentially related to making nuclear bombs, suggesting it is willing to compromise for a deal to win relief from harsh economic sanctions.
The official noted that no breakthroughs were achieved at the Geneva negotiations - and none had been expected.
"There is more work, much more work to do," the official added. "This is a beginning. Beginnings are rarely groundbreaking because you are putting pieces on the table."
"We are very clear that Iran must not acquire a nuclear weapon," the official said. "That's the outcome we're seeking to achieve."
Tehran denies allegations by Western powers and its allies that it is seeking the capability to produce atomic bombs.
The official said the U.S. administration will consult with partners and allies abroad and Congress at home, adding that if a good agreement with Tehran can be reached, Congress will likely approve it.
U.S. officials have said that they need Iran to increase the transparency of its nuclear program, stop enriching uranium to 20 percent, reduce its uranium stockpiles and take other steps to assure the world it does not want atomic weapons.
The senior official said that Washington was also concerned about Iran's Arak heavy-water nuclear reactor under construction that, once completed, could yield plutonium for atomic weapons.
"We have concerns about Arak and we will address those concerns in the negotiations," the official said.
There will be follow-up talks between Iran and the five permanent U.N. Security Council members - United States, Britain, France, China and Russia - and Germany on Nov. 7-8.
The U.N. Security Council has demanded that Iran halt uranium enrichment and other sensitive atomic activities. Tehran's refusal to comply with council resolutions has led to crippling U.N., U.S. and European Union sanctions.