The United States and the European Union have been studying Iran's response to a "final" draft agreement on reviving the landmark accord for the past week.

"What is important is the delay by the US side for issuing its response," Iran's foreign ministry spokesman Nasser Kanani said.

"We acted in this regard in a timely manner and always showed that we are responsible," he told reporters in Tehran.

"The US government is responsible for the current situation, and if it shows its political determination in action seriously and alongside promise and comment, acts responsibly, we can move toward the next stage."

Since taking office in 2021, President Joe Biden has sought to return the US to the deal that was unilaterally abandoned by his predecessor Donald Trump in 2018.

Talks on reviving the accord started in April 2021, and took a pause following the election of ultra-conservative Ebrahim Raisi as Iran's president in June that year, before resuming in November.

The 2015 agreement between Iran and six world powers -- Britain, China, France, Germany, Russia and the United States -- gave the Islamic republic sanctions relief in exchange for curbs on its nuclear programme.

The deal formally known as the Joint Comprehensive Plan of Action, or JCPOA, was designed to prevent Iran from developing a nuclear weapon -- something it has always denied wanting to do.

The British, French, German and US leaders spoke about the Iran nuclear issue during a phone conversation on Sunday, according to a White House statement.

They "discussed ongoing negotiations over Iran's nuclear programme, the need to strengthen support for partners in the Middle East region, and joint efforts to deter and constrain Iran's destabilising regional activities," the statement said.