"Today marks an important milestone toward preventing Iran from obtaining a nuclear weapon and ensuring its nuclear program is exclusively peaceful going forward," U.S. President Barack Obama said in a White House statement.

In a memo, he directed the secretaries of state, treasury, commerce and energy "to take all necessary steps to give effect to the U.S. commitments with respect to sanctions described in (the Iran deal)."

Several senior U.S. officials, who spoke to reporters on condition of anonymity, said actual sanctions relief for Iran was at least two months away.

Sunday was "adoption day" for the deal, which came 90 days after the U.N. Security Council endorsed the agreement reached by Iran, the United States, Britain, France, Germany, Russia and China under which most sanctions on Iran would be lifted in exchange for limits on Tehran's nuclear activities.

Secretary of State John Kerry said Iran would now have to act to restrain its nuclear program.

"These waivers will not take effect until Implementation Day, after Iran has completed all necessary nuclear steps, as verified by the IAEA (International Atomic Energy Agency)," he said in a statement. "If fully implemented, (the deal) will bring unprecedented insight and accountability to Iran’s nuclear program forever."

In Brussels, the European Union on Sunday published legal acts that open the way for the bloc to lift sanctions if Tehran meets the conditions tied to the landmark nuclear agreement.

Iran told the IAEA on Sunday it would fulfill a commitment under the deal to implement the Additional Protocol to its Comprehensive Safeguards Agreement, allowing U.N. nuclear inspectors more intrusive access to Iranian facilities.

"IMPLEMENTATION DAY"

Iran will take that step on "implementation day", the IAEA said in a statement. Under the deal, that is when the agency is due to have verified that Tehran has implemented restrictions on its nuclear activities and sanctions should be lifted.

Kerry said Tom Shannon, the appointee for Under Secretary of State for Political Affairs, and the U.S. point-man on Iran, Stephen Mull, would join senior officials from the six powers, Iran and the European Union in Vienna this week to discuss implementation of the deal.

In addition to Washington's conditional orders to suspend U.S. nuclear-related sanctions, U.S. officials said the United States, China and Iran were re-emphasizing their commitment to 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 in the nearly two years of negotiations that led to the July agreement.

Other steps Iran must take include reducing the number of uranium-enrichment centrifuges it has in operation, cutting its enriched uranium stocks and answering U.N. questions about past activities that the West suspects were linked to work on nuclear weapons.

Kerry noted that the IAEA had already said Iran had met its obligation to provide answers and access to the agency.

However, one U.S. official suggested on Saturday that the quality of answers Iran provides to the IAEA and the agency's assessment are not relevant when it comes to deciding on pressing forward with sanctions relief.

"That final assessment, which the IAEA is aiming to complete by December 15th, is not a prerequisite for implementation day," he said.

SOME SANCTIONS TO REMAIN

Tehran denies allegations from Western powers and their allies that its nuclear program was aimed at developing the capability to produce atomic weapons.

Unilateral U.S. sanctions against Iran not tied to its atomic program, such as those related to human rights, will remain even after the nuclear deal is implemented.

The U.S. officials were asked about Iran's decision to test a ballistic missile a week ago in violation of a U.N. ban that will remain in effect for almost a decade. The United States has said the missile was capable of delivering a nuclear warhead.

The officials reiterated that the launch was not a violation of the nuclear deal.

"This is not, unfortunately, something new," a U.S. official said, adding that the missile test should not be seen as an indicator of Iran's willingness to comply with the nuclear deal.

"There is a long pattern of Iran ignoring U.N. Security Council resolutions on ballistic missiles," the official said.

Washington has said it will seek Security Council action against Iran over the missile test. Once the deal is implemented, Iran will still be "called upon" to refrain from undertaking any work on ballistic missiles designed to deliver nuclear weapons for a period of up to eight years, according to a Security Council resolution adopted in July.

(Reporting by Louis Charbonneau; Additional reporting by Francois Murphy in Vienna; Editing by Paul Simao, Kevin Liffey and Alan Crosby)

© Thomson Reuters 2015 All rights reserved