Video report by ITV News Washington Correspondent Robert Moore
President Donald Trump has pulled America out of the Iran nuclear deal, describing it a as "horrible, one-sided deal" that "didn't bring peace" and "never will".
Mr Trump also announced in a televised address on Tuesday evening that he was imposing the "highest level" of economic sanctions on Iran.
He said: "The United States no longer makes empty threats. When I make promises, I keep them."
Iran's President Hassan Rouhani responded immediately, saying that there is only a "short time" to negotiate with the countries remaining in the deal.
Barack Obama, who signed the deal three years ago, has described Mr Trump's decision as a "serious mistake."
In a statement published on Facebook, he said:
"Walking away from the Joint Comprehensive Plan of Action turns our back on America’s closest allies, and an agreement that our country’s leading diplomats, scientists, and intelligence professionals negotiated.
"In a democracy, there will always be changes in policies and priorities from one Administration to the next.
"But the consistent flouting of agreements that our country is a party to risks eroding America’s credibility, and puts us at odds with the world’s major powers."
Speaking live on on Iranian state television, Rouhani warned his country could start enriching uranium more than ever in the coming weeks.
He said: "I have ordered Iran's atomic organization that whenever it is needed, we will start enriching uranium more than before." He says Iran would start this "in the next weeks."
In a joint statement, Prime Minister Theresa May, German Chancellor Angela Merkel and French President Emmanuel Macron said Trump's decision to pull out of the deal was a matter of "regret and concern".
France, Britain and Germany "regret" America's decision, calling it a threat to global efforts to contain nuclear weapons.
They said: "It is with regret and concern that we, the leaders of France, Germany and the United Kingdom take note of President Trump's decision to withdraw the United States of America from the Joint Comprehensive Plan of Action.
"Together, we emphasise our continuing commitment to the JCPoA. This agreement remains important for our shared security."
French President Emmanuel Macron tweeted that the "nuclear non-proliferation regime is at stake" because of Trump's announcement.
Mr Trump said the Joint Comprehensive Plan of Action (JCPOA) was a "great embarrassment" to him and warned it would lead to an "arms race" in the Middle East.
He said: "The Iran deal is defective at its core. If we do nothing, we know exactly what will happen."
He added: "In just a short period of time, the world's leading state sponsor of terror will be on the cusp of acquiring the world's most dangerous weapon.
"Therefore, I am announcing today that the United States will withdraw from the Iran nuclear deal."
The Iran nuclear deal is an agreement struck in 2015 between the UN Security Council's five permanent members - the US, UK, Russia, China and France, as well as Germany (known collectively as P5+1) - and Iran.
The P5+1 agreed to reconnect Iran to global markets by lifting tough economic sanction and in return the country agreed to significantly cut its nuclear development.
Prior to the deal being struck, Iran was progressing with a nuclear energy plan that world leaders feared could morph into a programme designed to manufacture nuclear weapons.
According to the White House, before 2015 Iran had enough enriched uranium and centrifuges to make 10 bombs.
Mr Trump said if Iran "continues its nuclear aspirations it will have bigger problems than it has ever had before" but said if Tehran wants a to negotiate a new deal then he is "ready, willing and able".
The president said it had been nearly 40 years since the regime "seized power and took a proud nation hostage" but said "great things can happen for Iran."
He added: "I want to deliver a message to the long-suffering people of Iran: The people of America stand with you."
Mr Trump also commented that Secretary of State Mike Pompeo is en route to North Korea ahead of the planned meeting with North Korean leader Kim Jong Un.
Trump said that Pompeo will be arriving in the country shortly and has meetings scheduled.
It will be Pompeo's second known visit to the country. Trump revealed last month that Pompeo also met with Kim over Easter weekend.