UPDATE 1-EU's Ashton offers to meet Iran on Dec. 5

* Ashton agrees to Dec. 5 meeting, suggests Europe

* Iran's Jalili had asked for talks in Istanbul

* If date, place agreed, talks could last several days

(Adds background, details)

By Luke Baker

BRUSSELS, Nov 12 (Reuters) - European Union foreign affairs chief Catherine Ashton has proposed to meet Iran's chief nuclear negotiator early next month to discuss Tehran's nuclear programme, an EU diplomat said on Friday.

Following consultations with the six world powers involved in negotiating with Iran -- the United States, Russia, China, France, Britain and Germany -- the diplomat said Ashton would write to Iran's chief negotiator, Saeed Jalili, proposing a meeting on Dec. 5, probably in Switzerland.

Jalili wrote to Ashton this week offering a meeting on either Nov. 23 or Dec. 5 in Istanbul.

"Catherine Ashton is expected to respond to Iran by the end of the day and is expected to agree to Iran's proposal to meet on Dec. 5, but not in Istanbul," the diplomat told Reuters, speaking on condition of anonymity.

"The meeting would be somewhere in Europe, most probably in Switzerland. A second, subsequent meeting may also be proposed for Istanbul," he said.

If talks are agreed -- they have been under discussion for nearly six months -- it would be the first time in more than a year that Iran has met to discuss its nuclear programme, which the world powers believe is aimed at developing atomic weapons but which Iran says is purely for peaceful purposes.

Pressure on Iran to return to negotiations has grown since the United Nations, the United States and the European Union began imposing tighter sanctions on Tehran in June.

President Mahmoud Ahmadinejad and others in the Iranian leadership have dismissed the sanctions, but there are indications that they are having an impact -- including a large fall in the value of Iran's currency -- and efforts on both sides to move toward talks have stepped up in recent months.

With the likelihood of a meeting growing, Ahmadinejad has reiterated in recent says that Iran will not negotiate over its right to develop nuclear technology, but has not entirely closed the door on a discussion of the uranium enrichment programme.

Western nuclear negotiators say that they have no problem with Iran having a civilian nuclear programme, but they want absolute reassurances that that programme is not being used as a cover to enrich uranium to a level capable of making weapons.

If Dec. 5 is fixed as the date for the meeting and a location is agreed, the talks could last for up to three days, EU diplomats have said, with representatives of each of the six world powers present.

Ashton, appointed EU foreign affairs chief last December, has been given a mandate by the powers -- who constitute the five permanent members of the U.N. Security Council plus Germany -- to explore negotiations with Iran.

She has said that all issues, including the uranium enrichment programme, should be on the table for discussion.

In letters since July, Jalili has not ruled out direct nuclear discussions, but has called for Ashton first to state her position on the nuclear weapons of "the Zionist regime", a reference to Israel, which does not confirm or deny that it has nuclear weapons.