Major powers are divided over what to put on the table should Iran resume talks on curbing its nuclear program and whether to allow it to continue enriching uranium to some degree, diplomats said on Friday.

Britain, China, France, Germany, Russia and the United States on Friday signalled their openness to fresh talks about Iran's nuclear program, which the West suspects is a cover to develop an atomic bomb but Iran says is to generate electricity.

EU foreign policy chief Catherine Ashton, who represents the group, issued a statement making clear that a diplomatic path remains open to Iran despite tougher sanctions and fresh speculation of a military strike on its nuclear facilities.

The EU3+3 has always been clear about the validity of the dual track approach, Ashton's spokesperson said in a statement that also formally released her October 21 letter offering to resume talks with Iran. We are waiting for the Iranian reaction.

The dual track refers to the combination of sanctions and diplomacy to try to curtail Iran's nuclear programs.

The release of the statement and the letter itself appeared be an effort to demonstrate that the major powers are willing to talk to Iran, while reiterating their demands that Tehran must return to the table willing to talk about its nuclear program.

It also appeared to reflect frustration at recent Iranian statements hinting at a willingness to return to the table but Tehran's failure to formally respond to the letter and commit to discussing the nuclear program in earnest.

NEGOTIATING STRATEGY UNCLEAR

If the Iranians were willing to resume talks, it is not clear what negotiating strategy the six major powers, which are known in diplomatic parlance both as the P5+1 and as the EU3+3 - might employ, notably on any confidence-building measures.

There is no agreement inside the P5+1 on how such confidence-building measures should or should not be presented to the Iranians, said one diplomat.

A central issue is whether the group might ask Iran to cease enriching uranium to the higher level of 20 percent but allow it, at least for a time, to continue enriching at lower levels -

a stance partly at odds with the group's past positions.

Uranium enrichment is a process that at low levels can yield fuel for nuclear power plants or, if carried out to much higher levels of purity, can generate fissile material for bombs.

Multiple U.N. Security Council resolutions have called on Iran to suspend its uranium enrichment and related activities, and the P5+1 group has taken the view that it must suspend such activities during any serious negotiation.

To permit Iran, even for a period, to enrich at lower levels would be something of a concession by world powers, although they have previously offered a temporary freeze-for-freeze in which Iran would halt expansion of its nuclear program and the major powers would not pursue additional sanctions.

Diplomats said the United States favoured the idea while others were more sceptical.

Asked why some members of the group might be willing to let Iran continue to enrich at lower levels, at least for a period, one diplomat said it reflected a desire to give diplomacy every possible chance to succeed.

That really is the crux of it. You want to be able to say that you pursued every option diplomatically to try to get Iran to halt its program, he said.

NO BACKSLIDING

Daryl Kimball of the Washington-based Arms Control Association said he understood that the United States and others in the group were prepared to propose a halt to enrichment at the 20-percent level and, probably, the removal of some or all of the stock of such material that Iran has already produced.

In exchange, the P5+1 would provide fuel for the Tehran Research Reactor, essentially updating a proposal it put forward in October 2009, before Iran was known to have begun enriching to the 20-percent level.

This proposal is being presented as an additional confidence-building step, Kimball said. It does not represent backsliding.

A diplomat said that proponents of the idea would argue that the nations negotiations with Iran had not softened their terms and that Iran must still suspend all its enrichment-related activities during any serious negotiation.

It's just an issue of sequencing, he said, reflecting the view of advocates. The counter-argument, he said, was that the Iranians might simply pocket such a proposal as a concession allowing them to continue enriching.

Diplomats have long said it made no sense to allow Iran to continue its suspect nuclear activities during talks because Tehran could stall the negotiations while perfecting its technology and advancing toward nuclear capability.

A senior Obama administration official told Reuters that if talks were to resume, the group would have a common stance.

If the Iranians accept the offer of the P5+1 to have talks on the basis of High Representative Ashton's October letter, we fully expect a unified P5+1 approach to the talks, the official said.

(Reporting by Arshad Mohammed; Editing by Ross Colvin and Xavier Briand)