A top Iranian politician on Tuesday expressed optimism over upcoming nuclear talks with world powers in Baghdad, but said there was still a "long way" to go in resolving Tehran's atomic issue.

"I am optimistic about the outcome of the talks in Baghdad, but I believe there is still a long way" to go to resolve the nuclear issue, said Mohsen Rezai, secretary of the Expediency Council tasked with advising Iran's supreme leader Ayatollah Ali Khamenei.

Rezai, the former commander of the elite Revolutionary Guards, said in remarks to Mehr news agency that the priority was to "build mutual trust" between Tehran and the world powers.

He also admitted that "the lifting of sanctions (imposed on Iran by the international community) is a long way" from happening.

The UN Security Council has slapped four rounds of sanctions on Iran for pursuing its nuclear programme, while the United States and the European Union have levied unilateral punitive measures on its oil and banking sectors.

The international community suspects that Iran, despite its repeated denials, is seeking to develop atomic weapons under the guise of a civilian nuclear programme.

Iran and the so-called P5+1 group -- comprising the five permanent UN Security Council members plus Germany -- revived talks on the nuclear issue in Istanbul last month.

Both sides described those talks as "positive" and are looking forward to the next round of discussions in Baghdad set for May 23.

Foreign ministry spokesman Ramin Mehmanparast reiterated Tuesday that Iran would "not give up its nuclear rights," when asked to respond to a report that the West has demanded Tehran stop enriching uranium to the 20 percent level.

"We believe we have rights under the Non-Proliferation Treaty and we will not give up those rights, at the same time we adhere to our commitments under the NPT," he said without explicitly responding to enrichment.

"The main view points of the 5+1 nations must be heard in Baghdad... it is clear that we will not abandon our right to master nuclear science for peaceful use," he added.

Several Iranian officials have hinted that Tehran may under certain conditions suspend its drive of enriching uranium to 20 percent level, which the United Nations has condemned.

The head of Iran's Atomic Energy Organisation, Fereydoon Abbasi Davani, said in April that Tehran had "no plans to continue enriching uranium to 20 percent for a long period."

Foreign Minister Ali Akbar Salehi followed him by saying there was "room for discussion" on the degree of enrichment, provided such rights are acknowledged.

Uranium enriched to 20 percent purity level can be used as fuel for nuclear reactors, but when refined to beyond 90 percent level, it can be used to manufacture atomic weapons.