US President Donald Trump has announced that the United States is effectively withdrawing from the Iran nuclear deal, defying last-ditch diplomatic efforts by his European allies to convince him otherwise.

"I made clear that if the deal could not be fixed, the United States would no longer be a party to the agreement," Trump said in a highly-anticipated address on Tuesday.

"The Iran deal is defective at its core. If we do nothing, we will know what exactly will happen.

"Therefore, I am announcing today, that the United States will withdraw from the Iran nuclear deal," he said.

Under the deal signed in Vienna with six world powers - the US, UK, France, Germany, Russia, China and the European Union - Iran scaled back its uranium enrichment programme and promised not to pursue nuclear weapons.

In exchange, international sanctions were lifted, allowing it to sell its oil and gas worldwide. However, secondary US sanctions remain.

The International Atomic Energy Agency (IAEA) has repeatedly confirmed that Tehran has been meeting its nuclear commitments fully.

A White House statement issued after Trump's speech said the US president directed his "administration to immediately begin the process of re-imposing sanctions" related to the deal, formally known as the Joint Comprehensive Plan of Action (JCPOA).

"The re-imposed sanctions will target critical sectors of Iran’s economy, such as its energy, petrochemical, and financial sectors."

'Foreign policy malpractice'

Trump's decision fulfills a campaign promise to cancel the 2015 pact, which he has repeatedly described as "the worst deal ever".

Responding to Trump's announcement, Iranian President Hassan Rouhani attacked Washington's "empty signatures" and said there was a short time to negotiate with the other world powers to keep the nuclear deal in place.

"I have ordered the foreign ministry to negotiate with the European countries, China and Russia in coming weeks. If at the end of this short period we conclude that we can fully benefit from the JCPOA with the cooperation of all countries, the deal would remain," he added.

Thomas Countryman, former US assistant secretary of state who helped negotiate the deal, said withdrawing from the agreement would further thrust the Middle East into the path of instability.

Countryman said with Trump's decision, the US becomes the first of the seven parties who is violating the agreement.

"That would be a serious case of foreign policy malpractice, and it would have several effects that would play out very slowly."

He also said the US is now left with diminished credibility to negotiate a better deal, while making negotiations with North Korea "more complicated".

International reaction

Hours before his speech, the White House said Trump had called French President Emmanuel Macron to discuss his decision.

Following the announcement, Macron took to Twitter to express the "regret" of Washington's European allies over the decision.

"France, Germany and the UK regret the US decision to leave the JCPOA," Macron said. "The nuclear non-proliferation regime is at stake."

In a joint statement earlier on Tuesday, the European Union, Britain, France and Germany said they met Iranian officials in Brussels and reaffirmed their support "to the continued full and effective implementation of the JCPOA by all sides".

Meanwhile, Russia warned on Tuesday that a "very serious situation" will emerge if Trump pulls out of the pact.

'Call for all-out confrontation with Iran'

Marwan Bishara, Al Jazeera's senior political analyst, called Trump's announcement "a major crossroad not only for the Middle East" but also for "international security".

"I've never seen the Middle East closer to the brink of war, a confrontation of sort, than it is today - at least not since the George W. Bush administration went against Iraq and tried to go against Iran."

Bishara said Trump did not only "alienate his European allies" but his announcement was "a call for an all-out confrontation with Iran" and "a total embrace of Israeli Prime Minister Benjamin Netanyahu's and of Israel's logic for the Middle East".

"In fact, it's Trump who embraced Netanyahu's vision of a middle east where a confrontation, a showdown with Iran is necessary for peace and security," added Bishara.

For his part, Ali Fathollah-Nejad, an Iran expert at Brookings Doha and the German Council on Foreign Relations, told Al Jazeera that there is an incentive in Tehran to keep the deal afloat despite Trump's decision.

He said, "a great portion of the Iranian elite" who benefited from the post-deal business would want the deal to survive.

On the other hand, some hardline factions do not mind the deal's collapse, he said.

Since Trump assumed office in January 2017, he had taken several steps to block the deal.

In October, he refused to certify that Iran is living up to the accord. He also targeted several Iranian businesses and individuals with new sanctions.

On January 12, Trump announced he was waiving the US sanctions for the "last time". He said if his demands to "fix the deal" were not met within 120 days, the US would withdraw from the deal on or before the deadline.

PHOTO CAPTION

FILE PHOTO: Head of the Iranian Atomic Energy Organization Ali Akbar Salehi attends the lecture "Iran after the agreement: Hopes & Concerns" in Vienna, Austria, September 28, 2016. REUTERS
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