The US said Friday anything short of an immediate furlough of a dual citizen taken back to an Iranian prison, would be a violation of Tehran's commitments to the United Kingdom.

In a deal earlier this week that freed two British Iranian detainees, Tehran promised also to furlough environmentalist Morad Tahbaz, who holds both British and American citizenship.

Tahbaz's sister, Taraneh Tahbaz, in an interview with Reuters said her brother had been taken back to prison on Friday after being released on furlough on Wednesday.

Shortly afterwards, a spokesman for Britain's foreign ministry said it had been told by Iran that Tahbaz, 66, had been taken back to Evin prison “to fit an ankle bracelet” and that the British government hoped he would be allowed out in coming hours.

A State Department spokesperson, speaking on condition of anonymity, noted that Iran made a commitment to the United Kingdom to furlough Tahbaz.

"We are not a party to this arrangement but would join the UK in considering anything short of Morad’s immediate furlough a violation of Iran’s commitment. We are urgently consulting with the UK on appropriate responses," the spokesperson said.

"We continue to work night and day to secure the release of our wrongfully detained citizens, including US-UK citizen Morad Tahbaz," the spokesperson said.

Tahbaz was arrested in 2018 and sentenced to 10 years in prison for "assembly and collusion against Iran's national security" and working for the United States as a spy.