Joe Biden has given a pledge that if the US returns to the Iran nuclear agreement, it will only subsequently leave if Tehran itself clearly breaks the terms of the deal.

The US president made the commitment, which addresses one of Iran’s key negotiating demands, in a joint statement issued with Germany, France and the UK. The statement followed a meeting on the margins of the G20 in Rome attended by Biden; the outgoing German chancellor, Angela Merkel; the French president, Emmanuel Macron; and the British prime minister, Boris Johnson.

The key paragraph of a lengthy statement read: “We welcome President Biden’s clearly demonstrated commitment to return the US to full compliance with the JCPOA [joint comprehensive plan of action] and to stay in full compliance, so long as Iran does the same.”

The pledge was welcomed by Mikhail Ulyanov, the Russian ambassador to multilateral bodies in Vienna, where talks on the future of the Iran nuclear deal have been on pause since June.

Throughout the talks, Iran has been seeking an undertaking, ideally legally underpinned, that if Iran returns to the deal, future US administrations will not repeat the walkout of the previous president, Donald Trump, which was accompanied by the imposition of tighter economic sanctions on Iran’s financial institutions and political bodies.

Biden is hamstrung in the value of the guarantees that he can give to Iran because the nuclear deal is not a signed treaty endorsed by the US Senate, and he cannot bind the hands of future US administrations.

However, the Biden pledge, underwritten by the main three European powers, may be a sign that the US wants to create a more positive atmosphere ahead of the resumption of the Vienna talks that Iran has – after much pressure and delay – promised to restart at some point in November. There have already been six rounds of talks, but they broke off in June to allow the new Iranian government, led by the president, Ebrahim Raisi, to review its negotiating strategy.

The shift in the US tone came the week after a cyber-attack disabled Iran’s petrol stations, an attack attributed on Sunday by Brig Gen Gholamreza Jalali, the head of Iran’s civil defence organisation, to Israel and the US.

The west has become increasingly concerned that Iran’s delay is a subterfuge, and that Tehran has been using the pause to strengthen its stockpile of uranium, weaken the UN nuclear inspections process and become more familiar with the use of advanced centrifuges that can produce highly enriched uranium. The US is concerned that Iran’s “breakout” time for developing a nuclear weapon is reducing and that the value of returning to the deal is fast diminishing. It has been looking at alternative options if it decides that diplomacy will not work.

The Iranian foreign minister, Hossein Amir-Abdollahian, said in an interview published at the weekend that there was no need for the negotiation and the simplest solution was for Biden to issue an executive order saying he was returning to the nuclear deal and lifting sanctions.

Amir-Abdollahian said his administration was embracing a balanced foreign policy, a phrase implying that Iran is going to deprioritise relations with the west, including the main European powers.