VIENNA, April 9 -- Iran and six major states hold detailed discussion covering all the issues needed for a final deal on the country's disputed nuclear program, EU foreign policy chief and Iranian chief negotiator announced Wednesday in a joint statement.

Speaking to reporters, EU foreign policy chief Catherine Ashton said the third round of talks held in Vienna was substantive and covered all the issues needed to be addressed in the final deal.

However, "a lot of intensive work will be required to overcome the differences," said Ashton.

Iran and 6 major states, so called P5+1 group (Britain, China, France, Russia, United States plus Germany) are striving to clinch a comprehensive agreement by 20 July to end the decade old Iranian nuclear deadlock.

The head of Chinese delegation, Wang Qun, Director-General of the department of arms control of China's foreign ministry seems to be optimistic to the talk, who told Xinhua the third round of talk covered all the issues needed to be addressed, and discussed the existing differences.

Iran and P5+1 group made an interim agreement last November, according to which Iran would halt some disputed nuclear activities while western states provides some sanction relief in 6 months, buying time for negotiation.

A senior U.S. official said the talk has been already "at the half way point" in the comprehensive negotiation, adding the talk was "in depth", but the official is cautious about the final agreement, noting it is still unknown if the gaps could be bridged in the future difficult talks.

Officials who attended the negotiation said the next phase would be aiming at drafting the agreement, which could be tougher.

Iran's Supreme Leader Ayatollah Ali Khamenei said today that Tehran will neither give up nor slow down nuclear research and development activities as the nuclear talks continues.

"The negotiators should not accept any forceful rhetoric from the other side." He stressed.

In the negotiation, Iran wants the west to relief the crippling sanctions, while their counterparts concerns about Tehran's developing nuclear capability.

According to IRNA's report, Iranian Oil Minister Bijan Namdar Zanganeh on Monday said Tehran will increase its oil export in the current Iranian calendar year (started March 21, 2014), underlining that sanctions can not affect Iran's decision to maintain its OPEC share.

All parties seek to strike a long term comprehensive agreement by the expiring date of the interim deal, July 20.

Iran is accused by western states to developing nuclear weapon underground, which Iran denies, saying the nature of its nuclear plan is peaceful, rounds of talk has been held to resolve the decade old stand off.

Western states see a window of opportunities of breaking the deadlock after the relatively pragmatic Iranian president Rouhani took office.