TEHERAN (AFP) - Iranian President Mahmoud Ahmadinejad who was due to address a water security forum in Thailand has cancelled the trip because of his busy schedule, a member of the president's media office said on Saturday.

"Ahmadinejad is unable to attend the forum due to his volume of work ... instead the Iranian delegation will be headed by the Head of the Environmental Protection Organisation, Mohammad Javad Mohammadizadeh," Mohammad Sheikhan said, quoted by the ISNA news agency.

An Iranian embassy official in Bangkok had said earlier that Ahmadinejad would address the forum next week, a month before ending his second and final term as president.

Ahmadinejad's trip to the northern Thai city of Chiang Mai was scheduled as Iran gears up for elections and as world powers press the Islamic republic over its suspected nuclear programme and support for the Syrian regime.

The race for Iran's highest elected office has intensified since former president Akbar Hashemi Rafsanjani and top nuclear negotiator Saeed Jalili officially registered for the June 14 election.

Ahmadinejad has endorsed his controversial aide and ex-chief of staff Esfandiar Rahim Mashaei.

Nuclear talks between Iran and the UN atomic agency failed to reach a breakthrough yet again Wednesday, as the top US diplomat in separate six-party negotiations warned Washington's patience was wearing thin.

Iran has also come under pressure over the conflict tearing apart its close ally Syria and has repeatedly rejected calls for foreign intervention in the increasingly brutal war.