LAUSANNE, Switzerland (AP) — Officials say the U.S. and Iran have moved close to agreement on key aspects of a nuclear deal but some problems remain ahead of a target date for an initial agreement.

Two officials tell The Associated Press that Iran has tentatively accepted limits on machines it could use to enrich uranium to 6,000 — or even less — at its main site.

The officials also say Iran has agreed to ship out all enriched uranium it produces to Russia. Enriched uranium can be used to make the fissile core of a nuclear weapon.

They say problems remain on the length of any restrictions on Iran's nuclear program, monitoring Tehran's compliance and other issues.

The officials demanded anonymity because they are not authorized to discuss the confidential talks.

Meanwhile, Secretary of State John Kerry has cancelled plans to return to the United States for an event honoring his late Senate colleague Edward Kennedy in order to remain at the ongoing talks.

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