Washington, April 3 (IANS): 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.

"I made clear that we were prepared to resolve this issue diplomatically, but only if Iran came to the table in a serious way.

"Now, sanctions alone could not stop Iran's nuclear programme. But they did help bring Iran to the negotiating table."

Obama said: "Iran has met all of its obligations. It eliminated its stockpile of dangerous nuclear material. Inspections of Iran's programme increased. And we continued negotiations to see if we could achieve a more comprehensive deal.

"Today, after many months of tough, principled diplomacy, we have achieved the framework for that deal. And it is a good deal, a deal that meets our core objectives," he said.

"This framework would cut off every pathway that Iran could take to develop a nuclear weapon. Iran will face strict limitations on its programme, and Iran has also agreed to the most robust and intrusive inspections and transparency regime ever negotiated for any nuclear programme in history. So this deal is not based on trust, it's based on unprecedented verification," the US president added.

"Many key details will be finalised over the next three months," Obama said, while laying down the basic outlines of the deal that the negotiators were working to finalise.

"First, Iran will not be able to pursue a bomb using plutonium, because it will not develop weapons-grade plutonium. The core of its reactor at Arak will be dismantled and replaced. The spent fuel from that facility will be shipped out of Iran for the life of the reactor. Iran will not build a new heavy-water reactor. And Iran will not reprocess fuel from its existing reactors -- ever," Obama said.

"Second, this deal shuts down Iran's path to a bomb using enriched uranium. Iran has agreed that its installed centrifuges will be reduced by two-thirds. Iran will no longer enrich uranium at its Fordow facility. Iran will not enrich uranium with its advanced centrifuges for at least the next 10 years. The vast majority of Iran's stockpile of enriched uranium will be neutralised."

Obama added: "Today, estimates indicate that Iran is only two or three months away from potentially acquiring the raw materials that could be used for a single nuclear bomb. Under this deal, Iran has agreed that it will not stockpile the materials needed to build a weapon. Even if it violated the deal, for the next decade at least, Iran would be a minimum of a year away from acquiring enough material for a bomb. And the strict limitations on Iran's stockpile will last for 15 years.

Third, this deal provides the best possible defence against Iran's ability to pursue a nuclear weapon covertly -- that is, in secret. International inspectors will have unprecedented access not only to Iranian nuclear facilities, but to the entire supply chain that supports Iran's nuclear programme..." Obama said.

"If Iran cheats, the world will know it," he noted.

Obama warned leaders of Congress not to stop the deal. "If Congress kills this deal not based on expert analysis and without offering any reasonable alternative, then it's the United States that will be blamed for the failure of diplomacy," Obama said. "International unity will collapse."

A comprehensive deal is expected by the end of June.