Clinton said she was carefully monitoring the situation and condemned Iran's "harassment" of the diplomats.
"We are following the situation with great concern," Washington's top envoy said.
"We have noted the statements from the (European Union). We find that the harassment of embassy staff is deplorable and we will continue to support the (United Kingdom) in calling for their release," she said.
Britain confirmed Monday that five of the nine British embassy staff members arrested in Tehran had been released, but denounced the continuing detention of the four who remain imprisoned.
Iranian authorities had arrested the staff over their alleged role in stoking the unrest which followed the disputed presidential election.
Iran has singled out Britain as the chief culprit in what Tehran sees as meddling from foreign countries in its internal affairs following the election.
The charge is denied by Britain.
