John Kerry has said that nuclear talks with Iran in Vienna were “getting to some real decisions” and that he was hopeful of a deal.

The US secretary of state said, however, that the negotiators still had “a few tough things to do”. With the Russian and Chinese foreign ministers expected back in the Austrian capital on Sunday evening to rejoin Kerry and their counterparts from the UK, France, Germany and Iran, there were rising hopes that the elusive, but potentially historic, deal could be struck on Sunday or Monday.

On his return early Sunday afternoon, the French foreign minister, Laurent Fabius, said: “I hope we’re arriving finally at the last phase of these marathon negotiations. I believe we are.”

The optimism was tempered by knowledge there have been three missed deadlines in the past two weeks, and the talks seem to be stuck on the intractable question of whether the UN arms embargo on Iran and restrictions on its missile programme should be left in place as part of any agreement.

The basic bargain underlying the deal – Iran accepts strict curbs on its nuclear programme in return for relief from economic and financial sanctions – appears to have been agreed and drafted in a text of 20 pages, and five annexes taking up another 60 pages. The arms and missile issue were left until the endgame in Vienna.

“The missile issue now seems to be the only key matter blocking the way to a deal,” according to Ariane Tabatabai, writing in the Bulletin of the Atomic Scientists.

Kerry attended mass at Vienna’s 14th century cathedral, where Mozart got married, before making his way to the Palais Coburg hotel, where the talks are taking place. He entered the venue through the back door on the crutches he has been using since a bicycle accident at the end of May.