EU foreign policy chief Catherine Ashton, leading ongoing nuclear talks with Iran in Vienna, is considering asking foreign ministers to attend but no decision has been made, her spokesman said Wednesday.

Ashton "is thinking about when to engage ministers as we move forward but no decisions have been made as yet. It would be an opportunity for them to take stock of where we are in the process," spokesman Michael Mann told AFP.

On Tuesday Iranian news agency IRNA reported that foreign ministers from the five permanent members of the UN Security Council -- Ashton is the chief negotiator -- would arrive in Vienna later this week.

It quoted an unnamed diplomat as saying that they would travel to the Austrian capital "probably" on Friday, Saturday and Sunday, but that this "does not mean we have reached an agreement".

The talks, which began last week, are aimed at reaching a potentially historic accord under which Iran would reduce in scope its nuclear programme in order to kill off once and for all concerns that Tehran will one day get the bomb.

The deadline to reach an accord is July 20 when an interim accord struck in Geneva expires, although this can, if both sides agree, be extended by up to six months.

Foreign ministers including US Secretary of State John Kerry and Russia's Sergei Lavrov twice descended on Geneva in November to broker the interim deal, and their presence in Vienna at some stage is widely expected.

A senior US official said after the last round on June 20 that foreign ministers "may well" travel to Vienna.

She said this would happen when both sides have "reached the narrowing of the gaps to the place where very tough political decisions need to be made and need to be made at the level of a minister."