A popular Sunni Muslim cleric has provided grass-roots and financial support to a leading anti-American Shiite cleric, a rare example of cooperation across Iraq's sectarian divide that has alarmed U.S. officials for its potential to bolster festering resistance to the American occupation, senior U.S. and Iraqi officials say.

The ties mark one of the first signs of coordination between anti-occupation elements of the Sunni minority, the traditional rulers of the country, and its Shiite majority, seen by U.S. officials as the key to stability in postwar Iraq.

The extent of the cooperation remains unclear between Ahmed Kubeisi, a Sunni cleric from a prominent clan in western Iraq, and Moqtada Sadr, the 30-year-old son of a revered Shiite ayatollah assassinated in 1999. But ideologically and practically, it represents a convergence of interests between the two figures, who were left out of the Iraqi Governing Council named last month and, in their own communities, have emerged as influential if still minority voices of opposition to the four-month-old occupation.

Supporters of the two clerics acknowledged cooperation, but denied there was any financial support.

U.S. officials say they are especially worried that such cooperation will strengthen Sadr. U.S. officials were taken by surprise by the young cleric's rise to prominence and have remained publicly dismissive of his influence. But they privately acknowledge his support among the poorest and most alienated in cities such as Baghdad and Basra -- a constituency that has long played a role in Iraqi politics -- and express frustration over their inability to curb his influence at a time of growing criticism of U.S. reconstruction efforts.

"This is a political challenge, and it is a distraction, and it keeps the show from getting on the road," said a senior U.S. official in Baghdad, who spoke on condition of anonymity. "We cannot afford the distraction."

Kubeisi, a charismatic speaker and respected religious scholar, enjoys support in conservative Sunni regions as a political and spiritual leader. Since the fall of the Sunni-led Baath Party, he has emerged as one of a handful of figures seeking to speak on behalf of the Sunni community, which has been left largely leaderless and adrift since the war.

The senior official said reports of financial support from Kubeisi to Sadr -- widely circulating among Iraqi officials -- came from U.S. intelligence in Iraq. According to one report, Kubeisi provided Sadr with $50 million, though the official cautioned that it was "unevaluated intelligence."

"He's getting a lot of money from Sunnis. I can't put a figure on it, but it's really a lot of money," he said.

Maj. Rick Hall, the executive officer of the 1st Battalion, 7th Marines, said the support was confirmed to him by Iraqi sources, though he had no specific figure. He called the reports "very reliable."

"We feel very confident" that Sadr had meetings with Kubeisi and "we believe reports we are told are true, reports of him receiving financing," Hall said at the Marines' base in Najaf, one of Iraq's holiest Shiite cities.

A senior official with the 25-member Governing Council, who spoke on condition of anonymity, described the financing as "100 percent true" and said it was common knowledge among Iraqi politicians and parties on the council.

U.S. officials declined to say where the money was coming from, but the Iraqi official said he believed it came from private individuals in the Persian Gulf, whose conservative, Sunni Muslim states have viewed with anxiety the prospect of a Shiite-dominated government in neighboring Iraq. By supporting the most radical Shiite elements, he said, they hope to prevent a united Shiite front in the contest for postwar power.

U.S. and Iraqi officials offered different assessments of how Sadr's group may have spent the money. At least some of it, they said, appears to have gone to supporters, part of the social welfare that has proved remarkably effective with Islamic groups elsewhere in the Arab world.

Hall said he believes it has been used in part to bring supporters from Baghdad and other Sadr strongholds to the Friday prayers in Kufa, near Najaf. The senior Iraqi official said he believed money was going to powerful tribes in southern Iraq, long a key source of support for the competing ayatollahs who vie for influence and supporters from their base in Najaf.

The U.S. and Iraqi officials said they believe Kubeisi has also encouraged followers from the restive cities of Fallujah and Ramadi in western Iraq -- the region where he draws his greatest support -- to attend Sadr's Friday sermons in Kufa.

Those sermons, which have at times drawn tens of thousands of supporters over the past month, have served as a key public venue for Sadr. Wearing a white funeral shawl to signify his willingness to sacrifice himself, he has railed against the Governing Council, calling it a tool of the U.S. occupation that should be dissolved, and repeatedly urged the creation of a religious army, albeit unarmed.

Mustafa Yaacoubi, a spokesman for Sadr, denied the reports that Sadr has received money from Kubeisi. He said the group raises its funds entirely from religious taxes and then, only from inside Iraq. Another spokesman, Adnan Shahmani, has put the taxes at $65,000 a month.

Taghlib Alusi, a spokesman for Kubeisi, who is currently in the United Arab Emirates, also denied that money had gone to Sadr. "There's no truth to it," he said. Sadr "has a lot of money. There's no need for Sheik Ahmed to give it to him."

But Alusi acknowledged cooperation between the two, beginning with a meeting in Najaf in late April. He said Sadr had sent a delegation from Najaf to Baghdad two weeks ago to explore greater cooperation. In the interests of sectarian harmony, he said that Kubeisi has encouraged his followers to pray with Shiites, who traditionally worship in separate mosques.

"We are friendly and we are brothers," he said.

Beyond their roles as religious officials, Kubeisi and Sadr share little in background. Kubeisi, who had a long, if ambivalent relationship with Hussein, went into exile in the United Arab Emirates in 1999. He returned soon after Baghdad fell on April 9 and then electrified a crowd of Sunni Muslims with a speech that warned U.S. troops their time was limited in Iraq.

"You are the masters today," he said. "But I warn you against thinking of staying. Get out before we force you out."

Sadr, the son of Mohammed Sadiq Sadr, who was killed with his two sons by Hussein's government, has inherited at least part of his father's popular, largely youthful following. His group, dominated by junior clerics engaged in grass-roots work and community organizing, remains one of the few mass-based movements in Iraq and draws on the deeply resonant symbols of Shiite suffering and martyrdom. In the past month, he has become increasingly vocal in his opposition to the occupation.

Both Kubeisi and Sadr have preached unity among Shiites and Sunnis. Those divisions run deep in the history of Iraq, where the Sunni minority has long dominated and Shiites were often brutally repressed by Hussein.

Both have also run afoul of U.S. authorities. U.S. officials criticized Kubeisi's newspaper, Al Sa'a, when it published a report in June about soldiers raping two Iraqi girls. U.S. officials said the story was false. Last week, soldiers visited Kubeisi's office after the newspaper published a story -- disputed by them -- that said U.S. soldiers had killed six children in Baghdad's Shiite neighborhood of Kadhimiya.

The senior U.S. official said authorities were also on the verge of closing a religious and anti-Baathist newspaper they said belonged to Sadr. Last month, it published a list of 134 Iraqis, many of them former senior government figures and party officials. The list declared them "tails of Saddam's tyrannical regime and his gang who will be caught by our hands sooner or later" and promised "the worst torture." Yaacoubi denied the newspaper, "The Echo of Sadr," was published by Sadr's group.

In the broadest terms, the senior U.S. official said he worried that funding from Kubeisi would add to Sadr's ability to organize his supporters, creating what he called an obstacle to U.S. efforts to oversee a new Iraqi government and constitution.

In Basra, for example, a group linked to Sadr holds one-third of the seats on the local council. While it denied having any hand in riots there earlier this month, it nevertheless supported the protests and warned of more. In a statement, it also accused British troops who control the city of depriving the population of basic services as part of "the enemy's conspiracies and imperialist schemes."

"He's a populist, a critic and a rabble-rouser and he's gotten awful, awful close to the line," the senior U.S. official said of Sadr. He added, "If the Shiites end up in an eye-gouging, ear-biting dispute among themselves, that's going to be bad for them, and it will certainly retard the progress that is supposed to be accomplished at a time in Iraq when time is important."

Reluctant to act themselves, U.S. officials have turned to Iraq's most senior Shiite clerics, also known as mujtahids, who have privately dismissed Sadr as a figure with no religious standing but are hesitant to publicly criticize him. Traditionally, the clergy have sought to keep disputes among themselves, projecting an image of unity. Given Sadr's lineage from a long and storied clerical family and his street support, the clerics seem unwilling to pick a fight with a potentially unpredictable and even violent outcome.

"We're watching him and some of the big mujtahids are watching us and we're both hoping the other does something," said the U.S. official.

Yaacoubi, the Sadr spokesman, said U.S. officials had no reason to act against the group and accused occupation forces of trying to provoke them, most recently when a helicopter knocked down a religious banner in Baghdad last week. In sermons and statements, aware of the crackdown it might bring, Sadr's followers have assiduously avoided any call to arms.

"Until now, we can say our office hasn't trespassed any red lines," Yaacoubi said in the group's headquarters in Najaf, which sits along a winding alley near the shrine of Imam Ali, one of Shiite Islam's most revered figures.

Over the past month thousands have gathered in Kufa, outside the Shiite holy city of Najaf, for Friday prayers led by Moqtada Sadr, an anti-occupation cleric. Sadr is believed by American and Iraqi officials to have received support and funding from Ahmed Kubeisi, a Sunni cleric. "This is a political challenge, and it is a distraction," a senior U.S. official in Baghdad said of the unusual alliance between Sadr, in portrait, and Kubeisi.