U.S. officials "feel like we are getting a very good deal" if an agreement can be secured in Geneva to offer modest, temporary and reversible sanctions relief for Iran in exchange for testing whether Iran is prepared to take a first step toward rolling back its nuclear enrichment program, U.S. Ambassador to the United Nations Samantha Power said on Thursday. If the agreement does not work, "we can slap that back on," Power said of the sanctions regime imposed on Iran. "We are keeping the crippling sanctions architecture in place, including the one in our neighborhood here in New York, which has been so biting and so crippling, which is the multilateral sanctions architecture at the UN," she pointed out. Power, interviewed on CNN "New Day," said the reason the United States is inclined to go forward with a first step with Iran is because a deal would allow "a more intrusive (nuclear) inspections regime -- to be able to say do they even follow through on commitments to roll back, to dilute, to lower the (nuclear) stockpile." "It would be unrealistic to think that we could obtain trust at the end of a long negotiating process such that I would come here and present to you a comprehensive deal," Power said. "We think a first step is very important, again, precisely because we have to test this regime," she went on. Turning to Afghanistan, Power said the United States is seeking a bilateral security agreement "that enables us to continue to work together beyond the end of the combat role in Afghanistan (at the end of 2014)." "We have come a really long way in those negotiations," she said. "We are very close. It now stands with the loya jirga. -- So the proof of how we are getting along is how much they want us to stay and continue to assist in the train, equip and assist role," she added. A letter from President Barack Obama sent to Afghan President Hamid Karzai "tries to offer the reassurance that the United States, in whatever role it stays in -- and we are negotiating that, of course, training, equipping, assisting on counterterrorism -- will respect the rights and the privacy of the Afghan people."