Iran says five of the British embassy staff arrested in Tehran on Sunday have been released.
An Iranian government spokesperson says three others are being interrogated.
The embassy staff, who are Iranian, were accused of involvement in unrest following the disputed presidential election earlier in June.
British Foreign Secretary David Miliband demanded the release of all the staff still held and said his European Union colleagues had agreed to a "strong, collective response" to any such "harassment and intimidation" against EU missions.
Britain and Iran have already expelled two of each other's diplomats since the election on 12 June election, which returned President Mahmoud Ahmadinejad to power, and which the opposition says was rigged.
The disputed presidential election result has sparked mass protests in which 17 people are thought to have died.
The body which supervises elections, the Guardian Council, has begun a partial recount of votes.
But defeated opposition candidate Mir Hossein Mousavi says the election should be annulled.