Theresa May has told Iran's president of her "serious concern" about the detention of a British charity worker in Tehran as she met the leader in New York on Tuesday.

The Prime Minister urged Hassan Rouhani to free Nazanin Zaghari-Ratcliffe, who was detained at Imam Khomeini airport in April 2016.

The British-Iranian mother, who works for the Thomson Reuters Foundation, was sentenced to five years in jail after being accused of spying by Tehran's Islamist regime, a charge she vehemently denies.

Last month she was granted a three-day release from Evin prison but her request for an extension was denied and she was forced to say goodbye to her four-year-old daughter, Gabriella, and return to jail.

"The Prime Minister raised our consular cases in Iran, including that of Nazanin Zaghari-Ratcliffe," Downing Street said of the talks held on the fringes of the United Nations General Assembly.