BAGHDAD — Iran appeared to balk Wednesday at a detailed proposal presented by six world powers to address urgent concerns about its nuclear program, including a freeze on its enrichment of uranium that could be converted to bomb-grade fuel, because of what the Iranian side suggested was an insufficient easing of sanctions in exchange.

But after a long day of diplomatic negotiations, both sides agreed to keep talking into Thursday. A senior American official said that despite disagreements some common ground had been reached, suggesting that diplomats had extended the constructive atmosphere that has prevailed since the talks on Iran’s disputed nuclear program were resumed last month.

“We’re getting to things that matter,” said the official, speaking on condition of anonymity because of the delicacy of the talks. “Even if we disagree on the shape, we think there is the beginning of a negotiation.”

The six powers — the United States, France, Britain, Russia, China and Germany — exchanged proposals with the Iranian side, which presented what Iranian news reports described as a five-point plan containing both nuclear and nonnuclear elements.