Vienna, March 20 (RHC-Agencies) -- The "substantive" round of talks on Iranian nuclear issues touched upon some controversial problems, EU foreign policy chief Catherine Ashton and Iranian Foreign Minister Javad Zarif said on Wednesday in a joint statement after a two-day meeting in Vienna.

Ashton and Zarif said the "useful" talks covered core issues surrounding the Iranian nuclear issue, including uranium enrichment, heavy water reactor, civilian cooperation and possible relief of sanctions on Iran.

Western nations are demanding that Iran significantly scale back its nuclear program, while Iran wants the West to relieve sanctions imposed on the Islamic country.

The unaccomplished Arak heavy water reactor is a main concern of the western states, which have argued that once it goes into operation, the reactor could be used to produce fuel, plutonium, for making a nuclear bomb.

However, Iranian Foreign Minister Javad Zarif said that the Arak reactor is part of Iran's nuclear program and will not be closed down.

Iran and the six powers to the meeting, the United States, China, Russia, Britain, France and Germany, agreed that the next round of talks would reconvene on May 7th in Vienna, which would continue to discuss 'tough' issues.

Tehran has said that the talks could be tougher in the future, as the parties have to make a subtle agreement, which respects Iran's nuclear right while addressing the western nations' concern.

Iran's negotiators faced domestic pressure to make compromise in the deal, before the talks, 200 Iranian lawmakers issued a statement urging Tehran's negotiators not to withdraw from the rights of the Iranian nation.