Geneva ? The United States and other world powers said they made progress Thursday in high level talks with Iran, taking fragile but potentially historic first steps toward addressing the Islamic government’s drive for nuclear capacity.

The meeting brought together officials from the U.S., China, Russia, Britain, France, Germany and Iran for the first substantive talks in more than a year, and will be followed by a wave of diplomatic activity in coming weeks, including another high-level gathering later this month.

In addition, Iran will admit international inspectors within a few weeks to its new uranium enrichment plant, near Qom, whose existence was revealed just last week.

And this weekend, the top nuclear official for the United Nations will visit Iran to discuss a plan to send low-enriched uranium from Tehran out of the country to be processed for medical use. A meeting of international technical experts on that program also was scheduled for later this month.

Despite the agreements, U.S. officials and allied officials remained cautious, describing the deal as positive but “fragile,” and expressing concern that Iran may not make good on its promises. Yet they insisted the deal represented important progress at a critical moment, coming at a gathering for which expectations were low.

President Barack Obama said the talks marked a “promising beginning,” but warned that if Iran intended to simply use delay tactics, then the United States would move to international sanctions. “If Iran does not take concrete steps, we are not prepared to talk indefinitely,” he said. “Pledges of cooperation must be fulfilled.”

Iran’s Foreign Minister Manouchehr Mottaki said at the United Nations that the talks were “constructive,” and said Iran would be willing to take part in a “summit,” meaning a gathering involving heads of state.

The agreements reached Thursday marked a sharp shift from the escalating rhetoric of recent days, when U.S. officials and their allies threatened Iran’s reeling economy with new economic sanctions, and the threat of military attack by the Israelis has hung in the air.