The US will impose "major" additional sanctions on Iran in a bid to prevent the country obtaining nuclear weapons, President Donald Trump says. He said economic pressure would be maintained unless the leadership in Tehran changed course. "We're putting additional sanctions on," he told reporters. "In [some] cases we are moving rapidly." It comes after Iran announced it would exceed internationally agreed limits on its nuclear programme. The limit on its stockpile of enriched uranium was set under a 2015 nuclear deal with world powers. In return, relevant sanctions were lifted, allowing Iran to resume oil exports - the government's main source of revenue. But the US pulled out of the deal last year and reinstated sanctions. This triggered an economic meltdown in Iran, pushing the value of its currency to record lows and driving away foreign investors. Iran has responded by scaling back its commitments under the nuclear deal. "If Iran wants to become a prosperous nation... it's OK with me," Mr Trump said. "But they're never going to do it if they think in five or six years they're going to have nuclear weapons." "Let's make Iran great again," he added, echoing his campaign slogan from the 2016 presidential election. In a later tweet, Mr Trump said the "major additional sanctions" would come into force on Monday. President Trump may have had second thoughts about carrying out a military strike against Iran, but he is set to toughen economic sanctions - the very policy that has helped bring the two countries to the brink of war. Iran's economy is suffering badly and it is threatening to breach some of the terms of the nuclear agreement it reached with the major powers. Mr Trump is also calling for negotiations with Iran. Earlier this month, Secretary of State Mike Pompeo offered talks with no conditions, save that Iran start behaving like "a normal nation". But Tehran dismissed this as "wordplay". Mr Trump's new sanctions will do nothing to reduce these tensions. A diplomatic "off-ramp" in this crisis seems as elusive as ever.