WASHINGTON - US President Barack Obama signed an order Sunday directing his administration to begin issuing waivers to Iran nuclear sanctions - but the waivers will only go into effect once Iran meets its obligations under the agreement limiting its nuclear programme.

The president directed Secretary of State John Kerry to issue the waivers and to "take all appropriate additional measures to ensure the prompt and effective implementation of the US commitments" in the agreement.

The directive comes 90 days after the UN Security Council endorsed the historic nuclear agreement with Iran.

On Oct 13, the Iranian Parliament endorsed the details of the nuclear agreement that was struck with six world powers in July, paving the way for restrictions on Iran’s nuclear programme in exchange for the lifting of economic sanctions.

The presidential memorandum marks what's being called "adoption day" for the international agreement intended to roll back Iran's nuclear programme. The milestone, four administration officials said, is a mere formality, driven more by the calendar than by any action by Iran.

"(The) adoption day is a calendar-driven event and it’s the day at which all the parties begin to take the steps they need to make sure they take to get to implementation day," State Department spokesman John Kirby said. "And we’re not at implementation day; that’s a whole different purpose."

No date is set for implementation day. Under the agreement, formally known as the Joint Comprehensive Plan of Action, implementation will come only when the International Atomic Energy Agency certifies that Iran has lived up to its obligations to reduce its stockpiles of enriched uranium, dismantle two-thirds of its centrifuges, and halt construction of new nuclear facilities.

Western officials have said they expect that to take four to six months. Iran is motivated to act quickly, said one of the four senior administration officials, who spoke on condition of anonymity under ground rules set by the State Department.

The agreement, signed by the United States, the United Kingdom, France, Russia, China, Germany and Iran, requires Obama and the European Union to direct the issuance of waivers on adoption day. Even though they won't go into effect for months, the arrangement allows businesses to know what sanctions are being waived, another senior administration official said.

Most of the sanctions being lifted apply only to non-US citizens and companies doing business with Iran. Most sanctions will still apply to US citizens under separate sanctions imposed on Iran for its support of terrorism and human rights violations. But sales of civilian passenger aircraft and handicrafts - most notably carpets - will be allowed.

The sanctions against Iran are authorized by Congress but implemented via executive order. Obama can waive those executive orders after Democrats in the Senate filibustered a resolution that would have blocked the agreement last month.

Besides the United States, Europe is also preparing to lift the trade sanctions that have hobbled the Iranian economy on Sunday.

The procedure to lift the embargo began 90 days after the UN Security Council endorsed the accord signed in Vienna in July, a milestone referred to as "Adoption Day."

But foreign firms will not be able to resume ties with Iran's oil industry and banks right away - sanctions will remain in place until Iran fulfils its end of the bargain. The next stage in the process - "implementation day" - will only come when UN nuclear watchdog the IAEA confirms Iran has dramatically scaled back its nuclear program. Tehran will have to surrender or dilute the bulk of its enriched nuclear fuel stocks, dismantle most of its centrifuges and halt a reactor capable of making plutonium.

"If fully implemented, it will bring unprecedented insight and accountability to Iran's nuclear program forever," US Secretary of State John Kerry promised. "This is an important day for all of us and a critical first step in the process of ensuring that Iran's nuclear program will be exclusively for peaceful purposes."

The European Union's foreign policy chief Federica Mogherini and Iranian Foreign Minister Mohammad Javad Zarif were also expected to make "adoption day" statements.

Ali Akbar Salehi, the head of Iran's nuclear agency, was awaiting President Hassan Rouhani's order to remove thousands of centrifuges from sites at Natanz and Fordo. "What we need to accomplish is a huge task. We hope to start this week or next week," he told state television. Dismantling the centrifuges, which enrich uranium, was part of a July 14 deal, known as the Joint Comprehensive Plan of Action, between Iran and six world powers.

As well as slashing the number of centrifuges at Natanz and Fordo to around 6,000, Iran will have to satisfy the IAEA that its Arak reactor cannot be used for military purposes.

China has agreed to work with Iran and the United States to "modernize" Arak so that it cannot produce plutonium, which can be used in a bomb, US officials said.

Iran has always denied seeking a nuclear weapon, which would dramatically alter the balance of power in an already unstable and war-torn Middle East region.

"We will start our actions when the president gives the order," Salehi said of Rouhani, estimating that the work to comply with the JCPOA would take around two months.

With Iranian citizens restless for economic relief, Tehran has said it hopes "implementation day" will come quickly - in less than two months - but Washington is more cautious.

"For us it's important that it's done right, not that it's done quickly," a senior administration official told reporters. "We cannot imagine less than two months."

"Those waivers will be out and issued so people will know what will be getting waived but it won't actually take effect until Iran completes its steps," the official said.

On Monday, envoys of the deal signatories - Britain, China, France, Germany, Russia and the United States - will meet in Vienna to form a commission to oversee the implementation of the JCPOA.

Meanwhile, Iran on Sunday notified the International Atomic Energy Agency that it would apply a protocol granting inspectors greater access to its nuclear sites, a further step in the implementation of a historic deal struck with world powers, the UN nuclear watchdog said.

"On October 18, the director general of the International Atomic Energy Agency was informed by the Islamic Republic of Iran that... Iran will provisionally apply the Additional Protocol to its Safeguards Agreement, pending its ratification by the (parliament)," the IAEA said in a statement.

The additional protocol is a voluntary agreement enabling the IAEA to gain greater access to Iran's nuclear facilities and research, to ensure that no atomic material is diverted to any covert weapons programme - an aim denied strenuously by Tehran.

The agency's chief Yukiya Amano is expected to provide a final assessment on the investigation by December 15.