The leader of Iraq's largest Shiite party was headed to Iran for lung cancer treatment after receiving the diagnosis at a Texas hospital, officials close to Abdul-Aziz al-Hakim said on Sunday.

The development was expected to create disarray in the Supreme Islamic Council in Iraq, the powerful political organization the United States has counted on to help push through critical, benchmark reforms.

Al-Hakim flew to the United States on Wednesday for tests after doctors at a U.S.-run hospital in Baghdad detected signs of cancer in one of his lungs. The diagnosis was confirmed at the University of Texas M.D. Anderson Cancer Center in Houston, officials in the al-Hakim organization said.

Al-Hakim, leader of the Supreme Islamic Council since 2003, left the United States early Sunday for Iran, where he will undergo chemotherapy treatment, according to the officials, who spoke on condition of anonymity because of the sensitivity of the subject.

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Iran, Iraq's Shiite neighbor to the east, hosted al-Hakim in exile for more than two decades during Saddam Hussein's rule. His party, formerly known as the Supreme Council for the Islamic Revolution in Iraq, was founded there with Iranian help in the early 1980s.

Al-Hakim, who gave up a 30-year smoking habit last year, has been coughing and suffering from high blood pressure for months, the officials said. His cough became considerably worse during two days of party meetings this month, where he was re-elected party leader.

News of al-Hakim's diagnosis came only hours after another key Iraqi leader, 73-year-old President Jalal Talabani, flew to the United States for a medical checkup. He was hospitalized in Jordan three months ago after collapsing. Doctors said he was suffering exhaustion and dehydration from lung and sinus infections.

Talabani, a Kurd and a close ally of al-Hakim, was expected to be away from Iraq for three weeks.

Al-Hakim, 57, wears clerical robes and a turban although he abandoned seminary studies years ago to concentrate on politics. He took over the leadership of the party after his brother and party founder, Ayatollah Mohammed Baqir al-Hakim, was killed in a massive bombing in the holy city of Najaf in August 2003.

Al-Hakim chose treatment in Iran rather than the United States because he wanted to be close to his family, party officials said. Proper treatment was not available in Iraq. Al-Hakim's choice of Iran also reflected his close links to the Shiite theocracy there.

Officials at the Texas hospital told al-Hakim that Shiite Iran and Sunni-ruled Saudi Arabia and Jordan were the countries nearest to Iraq where he could receive proper treatment.

Saudi Arabia and Jordan have often expressed concern that the empowerment of Iraq's majority Shiites had helped non-Arab Iran to expand its influence in the region and that Iraq's once-dominant Sunni Arab minority was being marginalized under Shiite rule.

Al-Hakim's absence could last several months or longer, said the officials, thus robbing Iraq from a key political player who has been a major partner in U.S. efforts to build a democratic system regardless of his ties to Iran.

American efforts to push through major reforms such as a new oil law, constitutional amendments and expanded opportunities for Sunnis in government would face even more hurdles without al-Hakim's active support.

For his party, the impact of his absence is potentially more dramatic.

"We are concerned," said Hameed Moalah, a lawmaker and a senior official of the Supreme Council.

"We are in a difficult situation," he said of al-Hakim's expected absence.

Al-Hakim's cancer was diagnosed a week after his party completed a two-day conference that signaled a departure from its old doctrines, adopted when the party was created 25 years ago vowing to topple Saddam's regime.

It drafted a new one that speaks of democratic values, freedom, human rights and national reconciliation.

Redha Jawad Taqi, a senior party leader, said Vice President Adil Abdul-Mahdi was running the party in al-Hakim's absence. Longer-term questions about succession at the top of the party remain unanswered.

Al-Hakim's son, 35-year-old Amar, has for years been groomed to succeed his father, but there were no plans for him to assume a leadership role for another decade, Taqi said.

"He is effective and energetic, but not yet powerful enough to take his father's place," he said, explaining that the leadership of the party was likely to stay in the al-Hakim family, one of Iraq's most prominent Shiite families.

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