U.S. officials predict it will take Iran months to curb its nuclear program as required under a historic nuclear deal.
The unnamed officials were speaking on October 18, so-called "adoption-day" for the agreement, which comes exactly 90 days after it was reached in Vienna after nearly two years of negotiations involving Iran, the United States, and five other world powers.
As part of the nuclear deal, U.S. President Barack Obama on October 18 ordered is administration to take steps toward lifting sanctions on Iran, in accordance with the nuclear deal struck in July between six world powers and Tehran.
"I hereby direct you to take all necessary steps to give effect to the U.S. commitments with respect to sanctions," Obama said in a memorandum addressed to the secretaries of state, energy, commerce, and the treasury.
Obama directed U.S. Secretary of State John Kerry to "take all appropriate additional measures to ensure the prompt and effective implementation of the U.S. commitments" in the agreement.
Under the deal, sanctions on Iran are to be lifted gradually as Iran limits its more sensitive nuclear work.
Western countries have suspected Iran of using its nuclear program to secretly develop a nuclear weapons capability, something Tehran has long denied.
Under the agreement, Iran should begin to make major changes to an underground nuclear facility, a heavy-water reactor, and a site for enriching uranium.
Senior U.S. officials said that sanctions relief will only come after the UN's International Atomic Energy Agency (IAEA) has verified Iran's compliance with the terms of the deal. They predicted that process will most likely take more than the two months Iran has projected.
Speaking on October 18 in Tehran, German Foreign Minister Frank-Walter Steinmeier said sanctions against Iran would likely remain in place until at least January as world powers wait to see whether Tehran fulfills its commitments to curb its nuclear program.
"That definitely won't be the case before the end of January," the top German diplomat told reporters in the Iranian capital.
Steinmeier, who is in Tehran to attend a preliminary meeting of the Munich Security Conference and meet with Iranian officials, added, "Now the question is whether Iran shows that it can fulfill its commitments."
The U.S. officials also said Washington, Beijing, and Tehran would issue a joint statement on October 18 regarding the redesign and reconstruction of the Arak research reactor so that it does not produce plutonium.
The fate of the Arak reactor was one of the toughest sticking points during the negotiations between Iran, the United States, France, China, Russia, Britain, and Germany.
Separately, Iran told the IAEA on October 18 it would fulfill a commitment under the deal allowing UN nuclear inspectors more intrusive access to Iranian facilities.
The milestone on the nuclear deal come despite a series of provocative steps by Iran in recent weeks.
Forces from Iran are reported to be fighting inside Syria, although Tehran denies this, saying it has only advisers there.
And a revolutionary court convicted a Washington Post reporter who has been held more than a year on charges including espionage. The court has not provided details on the verdict or the sentence.
Also, Iran successfully test-fired a guided long-range ballistic surface-to-surface missile.
With reporting by AP, AFP and Reuters