Three-quarters of the talks involving Iran and six world powers in Geneva this week have dealt with Iran's nuclear program, the US State Department said Tuesday.

"About 75 percent of the meetings yesterday and today were on the nuclear issue. It is something that remains a forefront among our concerns," State Department spokesman Philip Crowley told reporters.

The talks in Geneva involve the United States, Russia, China, Britain, France and Germany.

Iranian negotiator Saeed Jalili voiced irritation with Catherine Ashton, the European Union's top diplomat, after she said the Iranian nuclear issue was at the heart of the talks.

Iran makes it a point of honor of saying that the talks focus on nuclear issues in general, including that of Israel's nuclear capacities.

Ashton played the role of mediator in the talks that were described as constructive.

Crowley also said William Burns, head of the US delegation in Geneva, and his colleagues "had a couple of opportunities for brief, informal discussions with the Iranian delegation," but he declined to say what was discussed.

Jalili refused a one-on-one meeting with Burns, a source close to the talks said.