One of several conservative candidates has dropped out of Iran's presidential election to back another conservative who is challenging moderate President Hassan Rouhani.

State television on Monday reported that Tehran Mayor Mohammad Bagher Qalibaf had dropped out of the race.

The report said Qalibaf had thrown his weight behind Ebrahim Raisi, believed to be a favourite of Supreme Leader Ayatollah Ali Khamenei.

Iranians go to the polls on Friday to pick a new president. With Qalibaf dropping out, there are now five candidates contesting, though others may drop out in the coming days.