The United States is preparing "alternatives" in case efforts to revive a deal to curb Iran's nuclear ambitions collapse, Secretary of State Antony Blinken said Tuesday, as expectations grow that talks will fail.

"We continue in this hour, on this day, to pursue diplomacy because it remains at this moment the best option, but we are actively engaging with allies and partners on alternatives," he said during a visit to Indonesia. 

Negotiations restarted on Thursday last week to try to revive the 2015 deal between Iran and world powers, which the United States withdrew from under Donald Trump in 2018.

Iran claims it only wants to develop a civilian capability but Western powers say its stockpile of enriched uranium goes well beyond that, and could be used to develop a nuclear weapon.

US President Joe Biden has said he is ready to return to the agreement and Iranian officials maintain they are serious about committing to the talks.

But Tehran has been accused of backsliding on progress made earlier this year and playing for time.

Blinken's remarks came after Biden recently said the United States was preparing "additional measures" against Iran.

In Tuesday's comments, the top US diplomat also referred to a recent statement by European countries involved in the talks, which he said noted that "time is running out, that Iran is still not engaged in real negotiations".

"Unless there's swift progress... the Iran nuclear agreement will become an empty shell," Blinken said, referring to the statement. 

British Foreign Secretary Liz Truss went further at the weekend, warning that the resumed talks were the Islamic Republic's "last chance to come to the negotiating table with a serious resolution".

"There is still time for Iran to come and agree this deal," she told a news conference.

The 2015 agreement has been disintegrating since Trump pulled out. The deal ensured sanctions relief for Iran in return for tight curbs on its nuclear programme, which was put under extensive UN monitoring.

Trump went on to re-introduce sanctions, prompting Tehran to start disregarding the deal's limits on its nuclear activities in 2019.

Recent rounds of talks have stumbled on which sanctions Washington is prepared to lift and guarantees demanded by Iran to protect against the prospect of a future US withdrawal.