The opening round of talks between Iran and six world powers on a long-term deal for Tehran to curb parts of its nuclear program in exchange for a gradual end to sanctions is expected to take place next month in New York, a US official said on Monday.

"It is our understanding that the first round of comprehensive negotiations will be in New York in mid-February with dates still being confirmed," US State Department spokeswoman Marie Harf said in an e-mail.

"New York ... has a similar support infrastructure to Geneva," Harf said. "We believe that United Nations and international support is important for work on a comprehensive agreement."

A senior Western diplomat told Reuters that the six powers were looking at the early part of the week of February 16, though the talks were unlikely to begin before February 18 due to a US holiday.

In Washington, congressional aides said an attempt to impose new sanctions on Iran over its nuclear program has stalled in the US Congress and lawmakers were discussing a weaker measure.

Reuters