The US secretary of state, John Kerry, is to hold a face-to-face meeting with his Iranian counterpart, Mohammad Javad Zarif, hours before the deadline for reaching a nuclear agreement.

Diplomats at the Vienna talks said that if a last-minute compromise was going to be put on the table to break the deadlock, it would be on Monday morning.

“If this is about brinksmanship, this is the brink,” one western diplomat said.

The deadline set by the seven nations at the negotiations over Iran’s nuclear programme is midnight (CET), but there is growing talk of that being extended.

The Chinese foreign minister, Wang Yi, flew in overnight to join the talks for the first time, announcing he had new proposals to bridge the gaps between Iran and the west over curbs on the Iranian programme and the lifting of economic sanctions.

Wang met Zarif for an early breakfast and then Kerry. The foreign ministers of all the countries negotiating with Iran – the US, UK, France, Germany, Russia and China – were due to hold a meeting to coordinate a final common position. Afterwards, they would be joined by Zarif for a plenary session that was expected to decide whether to extend the talks, or to try to at least reach a political agreement that would serve as a statement of intent to conclude a deal in the coming weeks.

Asked about the chances of an agreement of any sort on Monday, an Iranian official said: “I hope we can. It depends on the mandate of the other side. Iran is ready.”

On Sunday, the British foreign secretary, Philip Hammond, promised a “last big push”, but warned that negotiators in Vienna were still a long way apart.

Hammond was speaking at the end of an intensive weekend of talks that brought foreign ministers to the Austrian capital to try to achieve a breakthrough on the last scheduled day of nine months of negotiations.

Western diplomats said that achieving a comprehensive deal in the last 24 hours was looking increasingly difficult, but added that none of the seven countries was willing to allow the negotiations to collapse given the progress that had been made and the dire consequences of failure for Middle East stability.

They said the talks could be given extra time, and there was speculation that they would be resumed next month. But it was unclear whether the foreign ministers in Vienna would sign a framework agreement, leaving some details to be worked out later, as proof of progress. Another, less ambitious option, would be for them to issue a political statement of intent which resolved few, if any, of the remaining contentious issues, but which envisaged concluding a deal in the future.

Asked about the possibility of an extension to the talks, Hammond said: “At the moment we’re focused on the last push … to try and get this across the line. Of course if we’re not able to do it, we’ll then look at where we go from there, but at the moment everybody’s talking about how we are going to try and bridge that gap and move things forward with the Iranians.”

However, Hammond said, he didn’t want to offer false hope. “We’re still quite a long way apart and there are some very tough and complex issues to deal with but we’re all focused right now on trying to get that deal done.”

In another sign that a critical moment was at hand, the Saudi foreign minister, Saud al-Faisal, flew to Vienna on Sunday and held a meeting with Kerry on his jet at the city’s airport. Saudi Arabia is not a participant in the negotiations, but as Iran’s greatest rival in the Gulf, Kerry has sought to consult Riyadh at pivotal moments.