BRUSSELS/BERLIN - Senior officials from six world powers said on Friday they were disappointed Iran had not accepted proposals intended to delay its potential to make nuclear bombs, and urged Tehran to reconsider.

But the officials, from Britain, France, the United States, Germany, Russia and China, stopped short of specific discussion of further sanctions that could be imposed on Iran, a senior EU official said after the talks in Brussels.

We are disappointed by the lack of follow-up on the three understandings reached (in the proposed deal), the powers said in a joint statement, which said Iran had not agreed to a meeting before the end of October to discuss the nuclear issue.

We urge Iran to reconsider the opportunity offered by this agreement... and to engage seriously with us in dialogue and negotiations, said the statement read out by Robert Cooper, the EU official who chaired the meeting.

The International Atomic Energy Agency had brokered a plan under which Iran would send low-enriched uranium to Russia and France, but Tehran on Wednesday rejected the proposal.

Under the initiative, Iran was given the option of shipping some 75 percent of its low-enriched uranium out of the country for it to be converted into fuel plates for a Tehran reactor that makes isotopes for cancer treatments.

The six powers met after U.S. President Barack Obama warned there could be a package of sanctions against Iran within weeks.

A senior EU official said sanctions were discussed at the meeting in general, not specific terms. These things are about timing and this was not the right time, he said.

The statement said the six agreed to stay in contact on the nuclear issue and expected to hold another meeting soon.

We are talking about weeks, not months. It's a question of whether we meet before or after Christmas, an official said.

The six powers also said Iran's construction of a second uranium enrichment site violated U.N. resolutions and said the International Atomic Energy Agency would have to address the issue at a November 26-27 meeting.

U.N. WATCHDOG AGAINST MORE SANCTIONS

The head of the U.N. nuclear watchdog had urged Iran to accept the offer to process its enriched uranium abroad by the year-end, while advising Western powers against more sanctions.

Mohamed ElBaradei said the IAEA-brokered plan was a rare chance to defuse mistrust over its nuclear program.

I would hope definitely that we'll get an agreement before the end of the year, he told a news conference in Berlin. I believe frankly the ball is very much in the Iranian court. I hope they will not miss this unique but fleeting opportunity.

ElBaradei said he did not think Tehran's rejection of the proposal, which was not delivered in writing, was its final position, and he remained in touch with Iranian counterparts.

What I got is an oral response, basically saying we need to keep all the material in Iran until we get the fuel, he said. That to me is an extreme case of distrust.

The West and Tehran had let each other down in the past and Iran had every reason to be distrustful, he said, adding that more sanctions would probably make Iran more hawkish.

In my view sanctions are going to make things much worse, ElBaradei said.

World powers want to reduce Iran's low-enriched uranium stockpile below the amount needed -- if refined to high purity -- for a single atom bomb. Tehran has said it could do swaps of LEU for reactor fuel on its own soil, but prefers to buy the fuel from foreign suppliers rather than part with any LEU.

Western powers suspect Iran is using its nuclear program to develop nuclear weapons. Tehran says its nuclear ambitions are limited to the peaceful generation of electricity.

(Editing by Mark Trevelyan)

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