"There has been a viable deal on the table since March," they said in a joint statement.

"We regret that Iran has not seized the diplomatic opportunity to conclude the deal. We urge it to do so now."

Formally known as the Joint Comprehensive Plan of Action (JCPOA), the 2015 deal gave Iran relief from crippling economic sanctions in exchange for curbs on its nuclear activities.

But in 2018 then US president Donald Trump unilaterally pulled out of the pact and reimposed sanctions, prompting Iran to begin rolling back on its own commitments.

Talks to revive the deal began in April last year, but have stalled since March.

The UN atomic energy watchdog earlier Thursday said Iran was removing 27 surveillance cameras at its nuclear facilities, warning this could be a "fatal blow" to the negotiations.

"These actions only aggravate the situation and complicate our efforts to restore full implementation of the JCPOA," the joint statement said.

"We urge Iran to... cease its nuclear escalation, and urgently conclude the deal currently on the table to restore the JCPOA, while this is still possible."