US Secretary of State John Kerry has warned of significant differences between Iran and six world powers trying to reach a nuclear agreement, as he and three European foreign ministers added their weight to try to narrow the gap at talks in Geneva.

Officials with delegations at the negotiating table had expressed optimism about progress achieved in Thursday's full day of talks.

But comments from Mr Kerry and his counterparts from Britain, France and Germany after they arrived in Geneva yesterday clearly indicated some obstacles remain in the way of any first-step agreement offering sanctions reduction for nuclear concessions.

Mr Kerry arrived from Tel Aviv after talks with Israeli Prime Minister Benjamin Netanyahu where he tried to defuse Israeli concerns about the Geneva talks.

Israel is strongly critical of any deal that even slightly lifts sanctions unless Iran is totally stripped of technology that can make nuclear arms.

Despite the lack of confirmation that an accord is within reach, Mr Netanyahu said he "utterly rejects" the "bad deal" he said was in the making.

In Geneva, Mr Kerry suggested it was too early to speak of any deal. He said "important gaps still remain".

He offered no details but in earlier comments to Israeli television, suggested Washington was looking for an Iranian commitment to stop any expansion of nuclear activities that could be used to make weapons, as a first step.

Six powers - the negotiators also include Russia and China - are considering a gradual rollback of sanctions that have crippled Iran's economy. In exchange they demand initial curbs on Iran's nuclear programme, including a cap on enrichment.

As a first step, the six have discussed ending a freeze on more than £31 billion in overseas accounts and lifting restrictions on petrochemicals, gold and other precious metals.

However, their proposal would maintain core sanctions on Iran's oil exports and financial sector, as an incentive for Iran to work toward a comprehensive and permanent nuclear deal.

Tehran could be pressing for more significant relief from the sanctions as part of any first-step deal.

The decision by Mr Kerry and his European counterparts to fly to Geneva comes after signs the global powers and Iran were close to a first-stage deal.

Russian officials said Foreign Minister Sergey Lavrov did not plan to attend. There was no word from Beijing on any plans by the Chinese Foreign Minister to join his colleagues.

French Foreign Minister Laurent Fabius, the first to arrive ahead of Foreign Secretary William Hague, spoke of progress, but told reporters "nothing is hard and fast yet".

Mr Fabius added: "I've come to Geneva to take part in the negotiations because the talks are difficult, but important for regional and international security. We are working to reach an accord."

Israel has been watching the talks warily from the sidelines. It has frequently dangled the prospect of military action against Iran should negotiations fail to reach the deal it seeks - a total shutdown of uranium enrichment and other nuclear programmes Tehran says are peaceful, but which could technically be turned towards weapons.

Mr Netanyahu said yesterday: "I understand the Iranians are walking around very satisfied in Geneva, as well they should because they got everything and paid nothing."

Any agreement would be a breakthrough after nearly a decade of mostly inconclusive talks, but would only be the start of a long process to reduce Iran's potential nuclear threat, with no guarantee of ultimate success.