WASHINGTON: US President Joe Biden has ordered his staff to prepare “additional measures” if troubled talks over Iran’s nuclear programme, which resumed on Thursday in Vienna, fail to reach a resolution.

“The president has asked his team to be prepared in the event that diplomacy fails and we must turn to other options,” White House Press Secretary Jen Psaki told reporters.

“We will have no choice but to take additional measures,” she added.

The latest round of talks began last week and were paused on Dec 3 after Western participants accused Iran of going back on progress made earlier this year.

International diplomats restarted the talks on Thursday for what the chair of the negotiations called the “difficult endeavour” of reviving the 2015 deal between Iran and world powers.

The heads of delegations from the parties to the 2015 deal -- Britain, China, France, Germany, Iran and Russia -- were present at the talks in Vienna.

An American delegation plans to take part indirectly in the coming days.

The heads of delegations from the parties to the 2015 deal — Britain, China, France, Germany, Iran and Russia — were present at Friday’s talks, which began at the Palais Coburg luxury hotel at around 12 pm (1100 GMT) and lasted a little more than an hour.

An American delegation plans to take part in the talks indirectly in the coming days.

“Delegations took a stack of the different consultations among capitals and they have come with a renewed sense of purpose to work hard,” Enrique Mora, the EU official chairing the talks, told the press after Thursday’s meeting.

Bilateral meetings as well as expert working groups are expected to continue this week.

Mora admitted that the negotiations were “a very difficult endeavour”, adding: “There are still different positions that we have to marry”.

Russia’s ambassador to the UN in Vienna Mikhail Ulyanov told the TASS agency that Thursday’s talks had “removed a number of misunderstandings that had created some tension,” but did not elaborate. The current round of talks is the seventh since they started in April. In June, Iran suspended them following the election of ultraconservative President Hassan Rouhani and they were only restarted on November 29.

US envoy Rob Malley “will plan to join the talks over the weekend,” said US State Department spokesman Ned Price on Wednesday.

“We should know in pretty short order if the Iranians are going... to negotiate in good faith,” Price told reporters, warning that “the runway is getting very, very short for negotiations.” For their part Iranian officials have insisted they are “serious about the talks”.

“The fact that the two sides are continuing to talk indicates that they want to narrow the gaps,” said Iran’s chief negotiator Ali Bagheri.