WASHINGTON (AFP) - The White House confirmed on Sunday that a nuclear agreement with Iran will take effect from Jan 20, but US President Barack Obama said he was under "no illusions" how hard it would be reach a comprehensive resolution.

"Beginning January 20th, Iran will for the first time start eliminating its stockpile of higher levels of enriched uranium and dismantling some of the infrastructure that makes such enrichment possible," a statement from the White House said.

Teheran earlier on Sunday gave the same start date for the landmark agreement that the Islamic republic clinched with world powers.

Under the deal reached in November, Iran agreed to curb parts of its nuclear drive for six months in exchange for receiving modest relief from international sanctions and a promise by Western powers not to impose new measures against its hard-hit economy.

Western nations and Israel have long accused Iran of pursuing a nuclear weapons capability alongside its civilian programme, charges denied by Tehran.

Mr Obama added in the White House statement: "With today's agreement, we have made concrete progress. I welcome this important step forward, and we will now focus on the critical work of pursuing a comprehensive resolution that addresses our concerns over Iran's nuclear programme.

"I have no illusions about how hard it will be to achieve this objective, but for the sake of our national security and the peace and security of the world, now is the time to give diplomacy a chance to succeed."