Talks between world powers and Iran on salvaging the 2015 nuclear deal will resume in Vienna on Monday after a five-month hiatus, but expectations of a breakthrough are low.

The talks could liberate Iran from hundreds of western economic sanctions or lead to a tightening of the economic noose and the intensified threat of military attacks by Israel.

The scale of Iran’s negotiating demands, ideological outlook of Iran’s new administration and western fears that Iran is covertly boosting its nuclear programme has created a sense of pessimism.

Joe Biden has offered to take the US back into the nuclear deal that Donald Trump left in 2018, but Iran and the US are in dispute over the precise US sanctions that must be lifted, and how Iran would reverse the multiple steps it has taken to build its nuclear programme in breach of the deal.

After a round of bilateral talks on Sunday, the formal talks will take place at the Coburg hotel between Iran, Russia, China, the UK, France, Germany and the EU on Monday afternoon. Iran has again said it will not hold talk direct talks with the US delegation.

Russia’s ambassador to the talks said the near 30-strong Iranian negotiating team was impressive and a good sign, but warned after the five-month delay that: “The talks can’t last for ever. There is the obvious need to speed up the process”. The bulk of the Iranian negotiating team remains unchanged even though the chief negotiator is now Ali Bagheri Kani, the deputy foreign minister and a hardliner that understands English, but not well enough to speak fluently.

The new Iranian regime says, at least rhetorically, it is coming to the talks not simply to pick up where the six previous rounds of talks ended, but to play new cards: including a demand for financial compensation from America for previous sanctions, and, even more problematic, for a guarantee that America will not leave the agreement again. The west regards both demands as unrealistic, and if seriously pursued in Vienna, the talks are doomed to failure. Biden has said if he rejoins the agreement his administration will not again leave, seen as the only guarantee he can make constitutionally.

Western diplomats admit they are unclear whether the new regime wants a deal or is playing for time covertly to strengthen its nuclear programme. Robert Malley, the head of the US negotiating team, said: “If that’s Iran’s approach, which is to try to use the negotiations as cover for an accelerated nuclear programme, and as I say, drag its feet at the nuclear table, we will have to respond in a way that is not our preference. Nobody should be surprised if at that point there is increased pressure on Iran”.

UK diplomats are reluctant to say Iran is now as little as four to six weeks away from the “breakout time” it needs to amass enough fissile material for a single nuclear weapon. The west is unclear partly because UN nuclear inspectors from the IAEA have been denied full access to the disputed nuclear sites, and talks last week to restore access broke down. Raphael Grossi, the International Atomic Energy Agency’s (IAEA) director general, again offered on Friday to return to Tehran, but has received no response. Iran is still many years from being able to weaponise its nuclear material, a goal it insists it is not seeking.

Should the Vienna talks collapse, the likelihood is the US and its allies will initially confront Iran at the IAEA next month by calling for an emergency meeting. Israel’s prime minister, Naftali Bennett, is building a coalition to take tough measures against Iran if the talks collapse.

Diplomats doubt Iran feels under sufficient economic or political pressure to rejoin the scheme but also point to the growing fortnight long protests over water shortages in Isfahan as a sign that internally Iran more fragile than it appears.

Lifting economic sanctions remains a popular objective inside Iran, but the current regime have done little to prepare the nation politically for the compromises that might be necessary. Omer Carmi, former visiting fellow at the Washington Institute, warns Iranian politicians are “implying to domestic audiences that Iran need not lift a finger at the negotiating table to secure sanctions relief”.

Ground covered but not resolved in the talks include the extent of sanctions relief, how to verify that sanctions have been lifted and how Iran expects European nations to respond if the US was to leave the agreement again.

Iran has long emphasised that it expects Washington to remove all sanctions that are “related to the nuclear deal”, including 1,500 individual sanctions.

In dispute are Trump-era sanctions from more than 500 individuals seen by the US as linked to human rights abuses or terrorism, including the 2017 Countering America’s Adversaries Through Sanctions Act (CAATSA).

On verification Tehran will be asked if it would allow an impartial body to supervise sanctions have been lifted, and what benchmarks would be required such as the country’s ability to buy/sell oil and transfer its foreign currency reserves.

Bagheri said he would also want European governments to guarantee they will trade with Iran and allow Russia and China to join any special trading vehicle.