Iran and six major powers are to keep negotiating past yesterday’s deadline for a long-term nuclear agreement as they tackle the most contentious issues, including the continuation of a UN arms embargo on Iran, the big powers said.

“We are continuing to negotiate for the next couple of days,” EU foreign policy chief Federica Mogherini said outside the hotel where the talks between Iran, Britain, China, France, Germany, Russia and the US are taking place.

The spokeswoman for the US delegation, Marie Harf, said the terms of an interim deal between Iran and the six would be extended through Friday to give negotiators a few more days to finish their work.

“We’re frankly more concerned about the quality of the deal than we are about the clock, though we also know that difficult decisions won’t get any easier with time,” Harf said. “That is why we are continuing to negotiate.”

The US and its allies fear Iran is using its civilian nuclear programme as a cover to develop a nuclear weapons capability. Iran says its programme is peaceful.

An agreement would be the most important milestone in decades towards easing hostility between the US and Iran, enemies since Iranian revolutionaries captured 52 hostages in the US embassy in Tehran in 1979.

A deal would be an important achievement for US President Barack Obama and Iran’s pragmatist president Hassan Rouhani, but both leaders face scepticism from powerful hardliners at home.

Russian Foreign Minister Sergei Lavrov said there was “every reason” to believe a deal would be done within “a few days”, and that there was an “understanding” that most of the current sanctions against Iran would be lifted.

“There is only one big problem in terms of sanctions – it is the problem of a weapons embargo,” he told journalists according to Russian news agency Interfax.

We’re frankly more concerned about the quality of the deal than we are about the clock

He said it was important to reach agreement on this as soon as possible, saying that “ending the bans on supplies to Iran of the weapons required to fight terrorism is a very, very relevant objective”.

It is the fourth time the parties have extended the terms of the interim deal, which was struck in November 2013 and provided Iran with limited sanctions relief in exchange for a halt to the production of uranium enriched to a purity level of 20 per cent.

The comprehensive deal under discussion is aimed at curbing Tehran’s most sensitive nuclear work for a decade or more, in exchange for relief from economic sanctions that have slashed Iran’s oil exports and crippled its economy. The latest extension to Friday leaves open the possibility that an agreement will not arrive in time for a deadline set by the US Congress for tomorrow in order to provide an expedited, 30-day review.

If a deal is sent to Congress between July 10 and September 7, Congress will have 60 days to review it, taking into account lawmakers’ August vacation. Obama administration officials fear that could provide more time for any deal to unravel.

“We are interpreting in a flexible way our deadline, which means that we are taking the time, the days we still need, to finalise the agreement,” Mogherini said, adding that there remained several difficult issues to resolve.

Among these, officials said, are Iranian demands for a UN arms embargo and ballistic missiles sanctions to be lifted, the timing of US and EU sanctions relief, and disagreements over future Iranian nuclear research and development. A senior US official said UN restrictions would remain both on Iran’s trade in arms as well as its access to missile technology but left open the possibility that these might be less onerous than they are at present.

UN restrictions on the development of Iran’s missile programme date to 2006. They call for Iran to abandon its ballistic missile programme and aim to prevent it from developing “nuclear weapon delivery systems,” which diplomats say covers any missile capable of delivering an atomic warhead.

While US Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif planned to remain in the Austrian capital to continue negotiating, the majority of the other foreign ministers planned to leave, some for only 24 hours.

US officials are loathe to ease the conventional arms embargo against Iran, fearing it would allow Tehran to provide greater military assistance to militants in Yemen, Syria or elsewhere in the Middle East.