Iran and six world powers struggled yesterday, after two days of talks, to overcome stumbling blocks to an interim deal under which Tehran would curb its nuclear programme in exchange for relief from economic sanctions.

Russian Foreign Minister Sergei Lavrov planned to join the talks in Geneva, a source in Moscow's delegation said. But there was no sign of ministers from the other five powers - the United States, China, France, Britain and Germany - following suit, something that could signal a deal was imminent.

The six foreign ministers earlier said some progress had been made during the first two days. But Iran's insistence that the six powers explicitly acknowledge its right to enrich uranium was proving an obstacle.

Iranian Foreign Minister Mohammad Javad Zarif, Tehran's chief negotiator, said significant headway had been made. "We are negotiating our differences and we have made considerable progress," he said.

Under discussion is an Iranian suspension of sensitive nuclear activities, above all medium-level uranium enrichment, in exchange for sanctions relief. That could involve releasing Iranian funds frozen in foreign bank accounts and allowing trade in metals, petrochemicals and aircraft parts.

If a preliminary agreement is reached for a six-month suspension of some of Iran's most sensitive nuclear activity, the six powers and Tehran will use that time to hammer out a longer-term accord.