Negotiators from Iran, the U.S. and five other world powers have reached agreement on a landmark deal to curb Iran’s nuclear program, diplomats here announced Tuesday.

The agreement, one of the most consequential and controversial international diplomatic achievements in decades, brings to a close nearly two years of talks, capped by a final, intense 18-day round of bargaining that lasted late into Monday night.

The agreement is designed to prevent Iran from developing a nuclear weapon for at least a decade.

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In a statement from the White House, President Barack Obama hailed the deal as an example of “American leadership.”

“Because America negotiated from a position of strength and principle, we have stopped the spread of nuclear weapons” to Iran, Obama said. “This deal is not built on trust, it is built on verification.”

By contrast, no deal would mean a greater chance of war, he said, vowing to veto any congressional effort to block the agreement.

“This deal offers the opportunity to move in a new direction” in the relationship between Iran and the rest of the world, Obama said. “We should seize it.”

Iranian President Hassan Rouhani similarly announced the deal to his country, in a televised address from Tehran. “Our prayers have come true,” he said.

A senior Iranian official in Vienna said he expects the United Nations Security Council to consider a new resolution that sets out the blueprint for the deal, and to adopt it in a week to 10 days.

“We believe this is a good agreement,” the official said. “There’s no need for spinning.”

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Announcement of the deal is all but certain to set off an intense debate within the U.S. and internationally.

Under its terms, Iran will accept a series of restrictions on its nuclear activities, some of which will last considerably longer than 10 years, and will allow inspection of known and suspected nuclear sites. In exchange, the U.S., Europe and the United Nations will agree to lift sanctions that have had a crushing impact on Iran’s economy.

Secretary of State John F. Kerry, who played a crucial role in the talks, said sanctions will only be lifted when Iran has taken a series of initial steps, including dismantling a heavy water reactor at Arak and shipping nearly all its enriched uranium out of the country.

The restrictions and inspections are designed to lengthen the so-called breakout period – the amount of time that Iran would need to produce a nuclear bomb if its leaders made the decision to race for one.

U.S. intelligence agencies believe Iran currently could achieve that goal in about two to three months. The deal aims to increase the breakout period to at least a year, which Obama says would give the U.S. time to respond – militarily, if necessary – to prevent Iran from getting a bomb.

The agreement aims to achieve that goal by limiting the number of centrifuges Iran can operate to enrich uranium, as well as the amount of uranium the country can stockpile. The inspection requirements are designed to prevent covert enrichment efforts.

U.S. officials said the deal came together late Monday night after 17 days in which the bargaining had veered between success and deadlock.

Kerry and other Western officials had hoped that foreign ministers could give the deal a final blessing Monday, but the last hours proved more difficult than expected, as officials from the seven countries involved – the U.S., Iran, Britain, France, Germany, Russia and China – wrangled over wording of the United Nations Security Council resolution that will eliminate earlier nuclear-related sanctions on Iran and set out the blueprint for the new agreement.

The Security Council is now expected to approve that resolution within weeks.

The final tradeoffs dealt with Iran’s desire – backed by Russia – to end the current U.N. embargoes on its trade in ballistic missiles and conventional arms. Under the agreement, the arms embargo will be lifted in five years, and the missile embargo in eight.

Both embargoes could be eliminated before that time if the U.N.’s nuclear watchdog agency, the International Atomic Energy Agency, determines that Iran’s nuclear program is entirely peaceful and in compliance with international rules. But such determinations take time.

Just before midnight Sunday, when ministers had signed off on the main elements of the 100-page agreement and its five annexes, Kerry phoned Obama at the White House. But the group was too weary for any cheering, a senior official said.

“There was not this sort of triumphalist celebration,” the official said, speaking on condition of anonymity to discuss the conversations.

The Obama administration regards the deal as a crowning achievement that resolves a security threat that could have led to war. Beyond that, a nuclear deal could potentially open the way for an end to decades of hostility between the U.S. and Iran, Obama has said. He has also said the deal would be worth having even if those additional benefits are not achieved.

Critics, including virtually all of the Republican presidential hopefuls, some Democratic lawmakers and officials in Israel and Saudi Arabia, regard Obama’s diplomacy as dangerously naive. A deal will simply strengthen Iran, they say.

Even if Iran complies with the deal’s terms, the country will emerge after a decade or more in a stronger position than ever and will resume its quest for nuclear weapons at that point, the critics argue.