Washington: US President Barack Obama said on Thursday that the world powers have reached a "historic understanding with Iran, which, if fully implemented, will prevent it from obtaining a nuclear weapon".

However, he said that the deal was a long time coming. "The Islamic Republic of Iran has been advancing its nuclear programme for decades. By the time I took office, Iran was operating thousands of centrifuges, which can produce the materials for a nuclear bomb -- and Iran was concealing a covert nuclear facility," Obama said.