(Illustration photo: REUTERS/Arnd Wiegmann)

Ambassador Nadine Olivieri Lozano and a small team returned to Tehran on Sunday. “The embassy will gradually resume operations," the ministry said in a statement.

The Swiss Embassy in Tehran plays a key role in US-Iran relations. Switzerland, a neutral country, has been mandated to represent US diplomatic interests in Iran since the US and Iran severed diplomatic ties in 1980.