Iran and six world powers left themselves with a lot to do by a July 20 deadline after a difficult fifth round of nuclear talks ended on Friday in Vienna.

The aim of the five permanent members of the UN Security Council plus Germany is to secure a mammoth deal by next month to reduce in scope Iran's nuclear programme and ease fears the Islamic republic will get atomic weapons.

Iran denies seeking to make a bomb and wants punishing UN and Western sanctions lifted. Neither Israel nor the United States have ruled out military action to stop Iran from getting a nuclear weapon.

The parties had "begun the drafting process" and would start the next round of talks on July 2, said a spokesman for EU foreign policy chief Catherine Ashton, chief negotiator for the six powers.

"We have worked extremely hard all week to develop elements we can bring together when we meet for the next round," said the spokesman, Michael Mann, after five days of discussions.

Officials on both sides said the drafting process had begun, but that haggling over language concerning the thorniest problems was being put off until later.

"We have not reached agreement on the main issues. In some cases, we can see light for agreement but in some others, there is none yet," Iranian Foreign Minister Mohammad Javad Zarif told Iran's media in the Austrian capital.

He said the draft document contained "more brackets than words", implying that many sections were far from finalised.

The senior US negotiator, Wendy Sherman, said that the talks had been "very tough but constructive" but that the draft document was still "heavily bracketed".

She added it was "still unclear whether Iran is really ready to take all the steps necessary to assure the world that its nuclear programme is and will remain exclusively peaceful."

"It has been another really tough round," said a diplomat from one of the "P5+1" powers late on Thursday, although he said this "doesn't surprise me or particularly dismay me".

Another diplomat said earlier this week that Iran was refusing to budge on most issues, calling this "worrying" and saying that there remained "major" differences on the key issue of uranium enrichment.

This process can make nuclear fuel for civilian purposes but also, when highly purified, for a nuclear weapon. It has been the main sticking point in negotiations with Iran for the past decade.

Western countries want Iran to slash the number of centrifuge enrichment machines in order to make it harder for Iran to process enough material for a bomb in a short period of time, if it chose to do so.

Other thorny issues include the duration of the mooted accord, the pace of any sanctions relief and a reactor being built at Arak that might give Iran weapons-grade plutonium.

- Extension -

The negotiations can be extended by up to six months beyond July 20, when an interim deal struck in November expires, but for now both sides were still aiming to get a deal by that date.

US President Barack Obama is particularly keen to ensure the deadline is met. He faces US midterm elections in November and hopes to silence accusations that the talks are merely giving Iran time to inch closer to the bomb.

This has been the long-standing accusation of Israel, the Middle East's sole if undeclared nuclear-armed state.

"We are absolutely focused on July 20 ... We are not interested in talking about a rollover," the P5+1 diplomat said, adding it would be a "long time" until such an extension is even discussed.

Mark Fitzpatrick, a former US State Department official now at the International Institute for Strategic Studies in London, said it was "not surprising" that difficult topics were being put off until later.

"If there is going to be a breakthrough on the key issues, it won't come until the last moment," Fitzpatrick told AFP.