* Iran's foreign minister blames headline for back pain
* Zarif to lead nuclear talks with world powers next week
* Incident points to domestic debate over ties with West
By Yeganeh Torbati and Jon Hemming
DUBAI, Oct 9 (Reuters) - Iran's foreign minister and chief nuclear negotiator went to hospital with pains he said were brought on by a hardline newspaper quoting him as saying President Hassan Rouhani's phone call with President Barack Obama was a mistake.
Mohammad Javad Zarif's brief visit to hospital is a pointer to the strength and possible rancour of the debate within Iran over the speed and extent to which the Islamic Republic should attempt to patch up its many quarrels with the West and the United States in particular.
Zarif is to lead his country's negotiating team in talks with six major world powers in Geneva next week, the first round of negotiations since Rouhani's election in June breathed new hope into decade-old talks on Iran's nuclear programme.
Iranian parliament speaker and former chief nuclear negotiator Ali Larijani described the talks as a "window of opportunity", telling reporters in Geneva the two sides should focus on confidence-building.
Rouhani, a relative moderate, and the U.S.-educated Zarif led a diplomatic drive to dispel distrust of Iran's intentions at the United Nations in New York last month. The trip culminated in the first phone call between the presidents of Iran and the United States since the 1979 Islamic Revolution.
Supreme Leader Ayatollah Ali Khamenei, often viewed as a hardliner, backed Rouhani's diplomatic overtures, but said some were "not proper", a possible reference to the call - giving the president's critics a chance to snipe at his initiative.
The hardline newspaper Kayhan said on Tuesday that Zarif had told a closed-door session of parliament's national security and foreign policy committee that Rouhani's conversation with Obama had been a mistake, as had the length of Zarif's own private meeting with U.S. Secretary of State John Kerry.
An infuriated Zarif denied saying any such thing, and said the newspaper report had affected him physically.
"This morning, after seeing the headline of one newspaper, I got severe back and leg pain. I couldn't even walk or sit," he wrote on his Facebook page late on Tuesday.
Cancelling several engagements, Zarif decided to rest at the ministry and held meetings with deputies "while resting".
"When four to five hours of rest did not solve the problem, at 5 p.m. I left my office ... and went to the hospital. Thank God, the MRI showed my problem was more due to nerves and a muscle spasm and will be solved through exercise," he wrote after returning from hospital some four hours later.
"In any case, it was a bitter but very informative day for me. I learned that whatever I want to say, to say it publicly, because otherwise the market for abuse is very active."
Without naming Kayhan, whose editor is appointed by Khamenei and is usually considered to reflect his views, Zarif said it was unfortunate that a confidential meeting in parliament had been leaked.
"Worse than this is that individuals who see themselves as the judges of my honesty, and who praise my honesty in a small headline, published with the biggest headline possible a quote attributed to me which does not conform with what I said."
Though Khamenei is the ultimate authority on all matters of state, especially the nuclear file, there is often a vigorous debate between subordinates in the government, parliament and security apparatus that frequently spills into public before the supreme leader gives his final word.
Western powers say Iran's uranium enrichment programme is aimed at achieving a nuclear weapons capability. Tehran denies the charge and says it only wants the technology for generating electricity and for medical research.