Geneva, April 1 (IANS): Negotiators scrambling to chart Iran's nuclear future seemed to be divided over several "key issues" which remain unresolved, casting doubts on earlier, more optimistic predictions that an agreement was ready to be drafted.

British Foreign Secretary Philip Hammond on Wednesday said progress has been made but characterised it as "slow going," the Washington Post reported.

"I'm optimistic that we will make further progress this morning, but it does mean the Iranians being willing to meet us where there are still issues to deal with," he told reporters. "Fingers crossed and we will hope to get there during the course of the day."

That contrasted with remarks by Russian Foreign Minister Sergei Lavrov after talks broke off in the overnight hours early on Wednesday. He said an understanding had been reached on all the major sticking points.

"I think we have a broad framework of understanding, but there are still some key issues that have to be worked through," Tass news agency quoted him as saying. "Some of them are quite detailed and technical, so there is still quite a lot of work to do, but we are on it now and we'll keep going at it."

Iran's Foreign Minister, Mohammed Javad Zarif, though less definitive, also sounded encouraging.

"The solution to most of the issues has been clarified," he told reporters. He said he hoped that "we can finalise the work and start the process of drafting".

However, Chinese Foreign Minister Wang Yi, who left the talks on Tuesday to return to Beijing, issued a statement on Wednesday suggesting that both sides still needed to give some time to narrow their differences.

"While each party has its own position, in this final phase, all parties must be prepared to meet each other halfway in order to reach an agreement," he said. "If the negotiations are stuck, all previous efforts will be wasted."

French Foreign Minister Laurent Fabius also left Lausanne on Wednesday, citing a previous commitment. He said he would return only if it was "necessary".

"Things have progressed, but not enough that we can reach an immediate deal," Fabius told reporters in Paris. "We are firm. We want a robust deal with detailed checks."

That leaves diplomats from only two of the five world powers still at the negotiating table alongside the US. Hammond and German Foreign Minister Frank-Walter Steinmeier are still at the talks, joining US Secretary of State John Kerry and Federica Mogherini, the foreign policy chief of the European Union.

The talks, which began in 2003 but picked up momentum only a decade later, have already produced tentative accords on dozens of issues.

For Iran, which insists it wants to use nuclear technology only for peaceful, civilian purposes, the primary goal is to get international sanctions lifted. The measures have been disastrous for its economy.

Despite progress at almost every stage of the talks, the final weeks have been consumed by negotiations over differences that are the hardest to bridge.

Iran wants sanctions lifted quickly, while the world powers are holding out for a more gradual easing. The US and its allies want restrictions to continue in the final five years of a 15-year accord that would monitor and limit Iran's nuclear programme, with research done on the country's outdated uranium-enrichment centrifuges.

It's still unclear what form a preliminary agreement might take. Iran is opposed to what would be in effect two separate agreements. Its supreme leader, Ayatollah Ali Khamenei, has insisted on only one, the final accord, so the US and its allies cannot "make things difficult" by challenging interpretations.