US Energy Secretary Ernest Moniz, an architect of the 2015 Iran nuclear deal, rejected Monday complaints from Tehran that Washington had not met its side of the bargain.
"The United States has done all that was asked of us... and more," Moniz said in Vienna where the accord was struck last July.
"The sanctions that were going to be relieved (under the deal) have been relieved."
The agreement came into force in January after decades of rising tensions.
Under it, Iran dramatically scaled back its nuclear activities, seeking to defuse concerns it would develop atomic weapons.
In return, major powers lifted sanctions imposed in previous years related to Iran's atomic activities.
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But others -- for example over Iran's missile programme -- remain in place.
On Thursday at the UN general assembly, Iranian President Hassan Rouhani accused Washington of blocking Iran's access to the international banking system, hurting its ability to conduct international transactions.
"They scare, they frighten the big banks with the threat of potential action by the United States Treasury. This is something that we oppose," Rouhani told reporters later.
Moniz, speaking at the UN atomic agency's annual general conference, said US officials "at their highest levels" have explained in Europe and to banks how to transact with Iran without falling foul of the remaining restrictions.
"The reality is that Iran has opened up quite a few correspondent relationships with banks, but they are smaller to medium-sized banks as opposed to the big global banks, which obviously is important for bigger transactions," Moniz told reporters.
He added that Iranian oil exports were "essentially" back to their pre-sanctions levels, giving Tehran "considerable extra cashflow", and that the country's economic growth has turned positive.
A day before Rouhani's comments, Airbus and Boeing said they had received US licenses to sell planes to Iran Air, clearing the way for the first Western aircraft sales to the country in decades.