VIENNA—Iran reached a landmark nuclear agreement with the U.S. and five other world powers, a long-sought foreign policy goal of President Barack Obama that sets the White House on course for months of political strife with dissenters in Congress and in allied Middle Eastern nations.

The accord, which comes after a decade of diplomatic efforts that frequently appeared on the verge of collapse, aims to prevent Iran from producing nuclear weapons in exchange for sanctions relief.

Obama’s administration and its partners hope the deal will resolve a dispute that at times threatened to spark a military conflict. In the optimistic view, it would ease tensions with Tehran over time and pave the way for fresh attempts to resolve some of the region’s many other conflicts.

Mr. Obama, in an address from the White House early Wednesday, called on Congress to approve the deal and threatened to veto any vote against it. "This deal demonstrates that American diplomacy can bring about real and meaningful change, change that makes our country and the world safer and more secure,” he said.

Critics in Washington, Israel and the Gulf nations that neighbor Iran say the deal will merely delay Iran’s path to nuclear weapons. After 10 years of restraint on its activities mandated by the agreement, Tehran will then be able to ratchet up its nuclear program and potentially unleash a nuclear arms race in the region, they fear.

Israeli Prime Minister Benjamin Netanyahu called the deal a "historic mistake.”

"Wide-ranging concessions were made in all of the areas which should have prevented Iran from getting the ability to arm itself with a nuclear weapon,’’ Mr. Netanyahu said. "The desire to sign an agreement was stronger than everything else.”

The agreement could provoke new strains in U.S. ties with its traditional Middle Eastern allies in Israel and the Gulf states led by Saudi Arabia. All have warned that lifting tight international sanctions will deliver an economic windfall that enables Iran to expand its regional influence by boosting funding for proxies in Syria, Lebanon, Yemen and elsewhere.

The last two years of diplomacy have represented easily the most intense dialogue between Washington and Tehran since diplomatic relations were ruptured after Iran’s 1979 Islamic Revolution.

In a televised speech, Iranian President Hassan Rouhani said the deal showed that "today a new page has started.”

The final round of negotiations stretched for more than two weeks and was punctuated by tensions and setbacks, at times devolving into shouting matches among international officials. The U.S. repeatedly warned it was willing to walk away from a bad deal while Iranians threatened to rev back up their nuclear program.

At the heart of the agreement between Iran and the six powers—the U.S., U.K., Russia, China, Germany and France—is Tehran’s acceptance of strict limits on its nuclear activities for 10 years. These are supposed to ensure that the country remains a minimum of 12 months away from amassing enough nuclear fuel for a bomb. After the 10-year period, those constraints will ease in the subsequent five years.

In exchange, the U.S., the European Union and the United Nations will lift tight international sanctions on Tehran, a move that Western diplomats say could help Iran’s economy to expand by 7% to 8% annually for years to come.

Iran, which analysts say could double oil exports quickly after sanctions are lifted, will also receive more than $100 billion in assets locked overseas under U.S. sanctions.

Mr. Obama has cast the nuclear diplomacy as an effort to avoid another costly, risky war in the Middle East. He recently said that even if the U.S. took military action against Iran’s nuclear facilities, it would only partially set back Tehran’s program, not eliminate it.

The nuclear agreement still faces significant hurdles before it takes full effect.

Iran must take an array of specific steps. It must disable two-thirds of its centrifuge machines used to enrich uranium, which can be used as fuel for nuclear energy or nuclear weapons. It must slash its stockpile of enriched uranium and redesign its nuclear reactor in the city of Arak so that it produces less plutonium, which can also be used in a weapon.

Oil-rich Iran has always insisted its nuclear program is for entirely peaceful purposes, such as producing electricity and medical isotopes.

After years of stalling, Iran also must disclose information on its past nuclear activities, which many Western officials believe was aimed at gaining nuclear weapons know-how. Iran must provisionally implement an agreement giving United Nations inspectors much broader access to sites inside the country and eventually get parliamentary approval for that agreement.

The nuclear deal will fan intense political debate in Washington, where Congress may vote within 60 days on the agreement. As a last resort, the Obama administration may have to rely on the support of Democrats to uphold a presidential veto if the Republican-led Congress votes to overturn the agreement.

Among other steps, the U.N. Security Council is supposed to annul past resolutions imposing sanctions on Iran and replace them with a new resolution. The U.S. is seeking to maintain limits on Iran’s arms trade and missile programs for a period in the new resolution.

Officials have said that if there are no major political or technical hurdles, Iran could complete its up-front commitments by year-end, paving the way for many financial, energy and commercial sanctions to be suspended or lifted. The U.S. will maintain sanctions on Iran linked to its rights abuses, ties to terrorist groups and to support for Syria’s regime among others.

However observers warned that given the complexity of the agreement, which contains one main text and five detailed annexes and totals about 100 pages, the risks of disputes over implementation of terms could cause delays or even derail the deal.

"The technical obstacles can be surpassed with goodwill and diligence, but political hurdles can turn into poison pills,” said Ali Vaez, senior Iran analyst at Crisis International, an international conflict resolution group.

"Neither Iran nor the U.S. has ever implemented such a complex quid pro quo. In an atmosphere of mistrust, misunderstandings are inevitable—thus the need for preserving the positive diplomatic momentum even after the deal is sealed.”

According to senior U.S. officials, the agreement will allow a Security Council ban on conventional arms sales to or from Iran to end after five years—or earlier if the U.N. nuclear agency gives its final, full all-clear that Iran’s nuclear program is purely peaceful. That is expected to take many years.

In addition, a ban on trading ballistic missiles and parts with Iran will expire after eight years unless the International Atomic Energy Agency gives its all-clear earlier. Iran is committed to using a special procurement channel to buy a wide range of products that could be used in a nuclear weapons program, the official said.

However, the agreement also includes specific oversight measures that few other countries have ever agreed to. There will be monitoring and oversight of Iran’s uranium mines, plants for manufacturing key parts of centrifuge machines and a range of activities that could be used to develop a nuclear warhead.

Wall Street Journal