The two countries will hold their first full-scale official direct meetings in decades on Monday and Tuesday in Geneva, with the route toward an eventual lifting of sanctions expected to be the main issue.
Abbas Araqchi, a vice foreign minister who will lead the Iranian delegation, said the tete-a-tete with the United States was essential, as the negotiations are delicately poised.
The P5+1 group of permanent members of the Security Council -- Britain, China, France, Russia and the United States -- plus Germany have long sought to reach a settlement over Iran's nuclear programme.
The announcement yesterday of the US-Iran meetings in Geneva came as a surprise, but appeared to confirm the need for secondary steps to close big gaps between Tehran and Washington's positions.
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"We have always had bilateral discussions with the United States in the margin of the P5+1 group, but since the talks have entered a serious phase, we want to have separate consultations," Araqchi said, quoted by official IRNA news agency.
"Most of the sanctions were imposed by the US, and other countries from the P5+1 group were not involved," he added, in a telling remark about how the US stance remains Iran's main concern.
The two Americans were part of a small team who through months of secret talks in Oman managed to bring Iran back to the P5+1 negotiating table last year.
Araqchi welcomed Burns's presence, saying he hoped it would be "as positive during these negotiations" as previously.