Nuclear deal with Iran is built on verification: Barack Obama

"This deal is not built on trust -- it's built on verification. Under this deal, we will, for the first time, be in a position to verify that Iran is meeting all of these commitments. International nuclear inspectors will have access to Iran's nuclear program -- where necessary, when necessary," Obama said.

"This is the most comprehensive and intrusive verification regime that we have ever negotiated. If Iran tries to divert raw materials to covert facilities, inspectors will be able to access any suspicious locations," he said.

He said: "This deal shows the real and meaningful change that American leadership and diplomacy can bring -- change that makes our country and the world safer and more secure."

Obama said he will veto any attempt by Congress to stop it.

"That is why it would be irresponsible to walk away from this deal. Moving forward, I welcome a robust debate in Congress on the details of this deal. As Commander-in-Chief, I am confident that this deal will meet the national security interests of the U.S. and our allies. So I will veto any legislation that prevents the successful implementation of this deal," he said.

In a historic nuclear deal, that Iran reached with six major world powers, Tehran has been granted sanctions relief in exchange for curbs on its nuclear programmes.

Media reports said the agreement aims to restrict Iran's nuclear work for more than a decade in exchange for the gradual suspension of sanctions that have hampered the country's oil exports, a major cornerstone of its economy.

Iran's Foreign Minister, Mohammad Javad Zarif, and E.U.'s Foreign Policy Chief Federica Mogherini are expected to read a joint statement in this regard.

The foreign ministers of Britain, China, France, Germany, Russia and the United States met for about an hour just after midnight to complete the agreement, which has been under negotiation for more than one-and-a-half year.

Iran has been a suspect for using its civil nuclear programme for developing nuclear weapons, though Tehran has assiduously denied it.