US President Barack Obama has signed an order directing his administration to take steps toward lifting sanctions on Iran in accordance with a July nuclear accord.

"I hereby direct you to take all necessary steps to give effect to the US commitments with respect to sanctions," Obama said in his order addressed to the US secretaries of state, energy, commerce and the treasury.

The order marks “adoption day” which, US officials said, is a mere formality as no sanctions will be lifted immediately.

Full relief will come on "implementation day" when the US could confirm that Iran has met its commitments under the so-called Joint Comprehensive Plan of Action (JCPOA), Obama said.

No date is set for implementation day. Western officials have said they expect that to take four to six months.

"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," US Secretary of State John Kerry said in a statement.

Kerry said US officials would join senior representatives from the P5+1, Iran and the European Union in Vienna this week to discuss implementation of the accord.

Obama directed 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.

In Iran, Ali Akbar Velayati, a top adviser to Leader of the Islamic Revolution Ayatollah Seyyed Ali Khamenei said, "On implementation, all should be watchful that Westerners, particularly Americans, keep their promises."

Velayati said Iran expects that the United States and other Western countries that negotiated the agreement will show their "good will" through lifting sanctions.

In Brussels, the European Union on Sunday published legal acts that open the way for the bloc to lift sanctions.

Iranian officials have repudiated the sanctions as illegal, designed to pressure the country for political reasons because its nuclear program represents nothing but peaceful intentions.