The US Secretary of State says Iran must "act like a normal country, or... see its economy crumble".Read more
Iran's President Hassan Rouhani has vowed to "break" swingeing sanctions re-imposed by the US targeting core parts of the economy.
The Trump administration is restoring all sanctions lifted under the 2015 nuclear deal in a bid to pressure Iran.
They will hit oil exports, shipping and banks, and make it difficult to do business with the oil-rich nation.
US Secretary of State Mike Pompeo said Iran must "act like a normal country, or see its economy crumble".
Theresa May will make a personal plea for the release of an imprisoned British-Iranian woman in Tehran during talks with Iran's president later today.
Charity worker Nazanin Zaghari-Ratcliffe was jailed for five years in 2016 after being convicted of spying, which she denies.
The PM will raise "serious concerns" about the detention of the mother-of-one and urge Hassan Rouhani to free her on humanitarian grounds.
Mrs May is expected to meet Mr Rouhani for talks in New York today, where she is attending the United Nations General Assembly.
Ms Zaghari-Ratcliffe's constituency MP Tulip Siddiq welcomed Theresa May's expected intervention.
Ms Zaghari-Ratcliffe was was arrested at Tehran airport after visiting her family on holiday. She insists the visit was to introduce her daughter Gabriella to her relatives.
The mother-of-one from Hampstead, north London, was recently reunited with her family, including her four-year-old daughter, during a temporary release.