The Obama administration said Tuesday it would review any new Iranian proposal on its nuclear program seriously and with mutual respect. Iran's nuclear negotiator said earlier Tehran has revised proposals and is ready to re-engage on the nuclear issue with major world powers.

The State Department says no new Iranian proposal has been conveyed to the United States or other countries that have been involved in nuclear talks with Tehran.

But U.S. officials are not dismissing the prospect of new talks with Tehran, and say a constructive Iranian response to a big-power nuclear offer earlier this year would be welcome.

Last April, the United States, along with other permanent U.N. Security Council member countries and Germany, the P5+1, offered Iran a revised package of economic and other incentives for Tehran to halt uranium enrichment and return to negotiations over its nuclear program.

The Obama administration, with backing from key European allies, has warned of tougher sanctions against Tehran if a negotiating impasse continued through end of this month.

Earlier Tuesday in Tehran, Iranian nuclear negotiator Saeed Jalili said Iran has developed a revised set of proposals on the nuclear issue and is ready to hold talks with world powers to ease common concern.

At a news briefing, State Department Spokesman Ian Kelly said no new Iranian ideas had yet been conveyed to the P5+1 but said they are ready to give any proposal serious consideration.

"We would review any proposal that they give us seriously and in the spirit of mutual respect. We would welcome the Iranian government's constructive response to the P5+1, to their April 2009 invitation to meet face-to-face," he said. "Moving forward,  these discussions could begin to bring Iran into compliance with its international obligations and create confidence in the exclusively-peaceful nature of its nuclear program," said Kelly.

Iran says its massive enrichment program is solely to produce fuel for civilian power plants, but U.S. officials, among others, believe Iran is trying to develop nuclear weapons.

The apparent Iranian overture for talks came as senior diplomats of the P5+1 prepared to meet Wednesday in Frankfurt on the nuclear issue. It is also expected to be a major topic on the sidelines of the U.N. General Assembly later this month in New York.

Under questioning here, spokesman Kelly said the conciliatory gesture from Tehran may well be just a maneuver by Tehran to try to diminish international support for tougher sanctions in advance of the Frankfurt meeting and New York discussions.

He said skepticism about Iranian motives was reinforced by a report last week by the International Atomic Energy Agency that Iran is continuing to expand its nuclear program and has refused to adequately account for past activity that appears weapons-related.

Kelly none the less said the P5+1 has offered Iran a path to becoming a full and respected member of the world community and said it remains to be seen if Tehran will choose that path.

On a related issue, Kelly said he has every reason to expect that Iranian President Mahmoud Ahmadinejad will be granted a visa to attend the U.N. General Assembly meetings in keeping with the United States' legal obligations as the U.N. host country.

Mr. Ahmadinejad has addressed the U.N. in each of the last several years. But there have been calls from some members of U.S. Congress to bar him this year because of the Tehran government's crackdown on crowds protesting the Iranian leader's disputed re-election in June.