For decades, neutral Switzerland has played a central role in maintaining basic diplomatic contacts between Iran and the United States.
But the country temporarily closed its embassy on March 11 days after the Middle East war erupted with the first US-Israeli attacks on Iran.
The Swiss foreign ministry said four staff had returned and insisted the channel of communication between the US and Iran had been kept open even while the mission was shut.
"The decision to gradually reopen the embassy was taken after a risk analysis and in consultation with Iran and the United States, whose interests Switzerland represents under its protecting power mandate," the foreign ministry said in a statement.
Switzerland "is prepared to provide its good offices if the parties so wish and supports all diplomatic initiatives that contribute to de-escalation and a lasting peace".
Switzerland has represented US interests in Iran since Washington broke off relations with Middle Eastern country after the 1980 hostage crisis, which came a year after the Iranian revolution.
The Swiss embassy in Tehran handles all consular affairs between the United States and Iran, including passport requests, altering civil status and consular protection for US citizens in Iran.
Under the protecting power mandate, "Switzerland can either offer to act as a go-between on its own initiative or can fulfil this function at the request of the parties concerned, provided that all those involved agree", the foreign ministry says on its website.
A ceasefire has been in place since April 8 but prospective peace talks between senior US and Iranian envoys Pakistan are hanging in the balance.
Iran has all but closed the Strait of Hormuz in retaliation for the war, while the US has imposed a blockade of its own on Iranian ports.
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