Dhaka, March 14 -- A letter US President Donald Trump wrote to Iran's supreme leader in an attempt to jump-start talks over Tehran's rapidly advancing nuclear program has arrived in the Iranian capital.

While the text of the letter hasn't been published, its arrival comes as Trump levied new sanctions on Iran as part of his "maximum pressure" campaign targeting the country. He also suggested military action against Iran remained a possibility, while emphasising he still believed a new deal could be reached, according to AP.

Iran's 85-year-old Supreme Leader Ayatollah Ali Khamenei has mocked Trump, but officials in his country also have offered conflicting signals over whether negotiations could take place.

Here's what to know about the...