Parties to the Iran nuclear agreement on Tuesday decided to "speed up" talks in Vienna, Tehran said, to revive the tattered accord.

The remaining partners to the 2015 deal have been engaged in negotiations since early this month to try to bring the United States back onboard.

Delegates from Britain, China, France, Germany, Iran and Russia met for less than two hours to kick off the third round of EU-chaired negotiations in a Vienna luxury hotel, according to diplomats.

"During the meeting the participants decided to speed up the process of talks," Iran's foreign ministry said in a statement.

Iran has refused to negotiate with the US directly, but US delegates are staying in an adjacent hotel and are being regularly updated by EU negotiator Enrique Mora's team in a round of shuttle diplomacy.

"Back in Vienna for the JCPOA Joint Commission, experts discussions and separate contacts with US in this third week of talks to advance on our objectives: US rejoining the JCPOA and full implementation of the deal," Mora wrote on Twitter, using the acronym of the agreement's formal name.

The 2015 accord aimed to give Iran sanctions relief in exchange for a curtailment of its nuclear programme, but the deal started to unravel in 2018 when then US president Donald Trump rejected it.

In retaliation to the US re-imposing sanctions, Iran has started to step up its nuclear activities since 2019.

Tehran has insisted on its readiness to return to its nuclear commitments once it is sure of sanctions relief, and US President Joe Biden has said he is ready to revive the agreement.

Major differences remain

Negotiators have lauded progress in the talks, but warn there is still a long way to go and details need to be worked out.

"We do see some important progress registered, while we also see some major differences remain at this critical juncture," Chinese negotiator Wang Qun told reporters after Tuesday's meeting.

Mora had said in a column published Monday on the Spanish site Politica Exterior that "many obstacles" remained, mentioning "domestic politics in Tehran and Washington, where the agreement probably has more detractors than supporters".

The hope is to achieve a concrete result "by the end of May", before Iranian presidential elections in June, a diplomat familiar with the discussions told AFP ahead of the latest round.

Last Thursday, Iran's foreign ministry issued a defence of its negotiating team, following days of growing criticism by state media.

A senior US official last week said Washington had shared details of the sanctions it was prepared to lift with Tehran.