By ADAM ENTOUS And MATTHEW ROSENBERG

Iran is secretly helping Syrian President Bashar al-Assad put down pro-democracy demonstrations, according to U.S. officials, who say Tehran is providing gear to suppress crowds and assistance blocking and monitoring protesters' use of the Internet, cellphones and text-messaging.

At the same time, communications intercepted by U.S. spy agencies show Tehran is actively exploring ways to aid some Shiite hardliners in Bahrain and Yemen and destabilize longstanding U.S. allies there, say U.S. officials familiar with the intelligence. Such moves could challenge interests of the U.S. and Saudi Arabia and inflame sectarian tensions across the Middle East, they say.

"We believe that Iran is materially assisting the Syrian government in its efforts to suppress their own people," said an Obama administration official.

U.S. officials say they don't see Iran as the driving force behind popular revolts against longtime U.S. allies in the Mideast, and caution they have no concrete evidence that Iran is providing or preparing large-scale financial or military support to opposition elements in Bahrain or Yemen.

Rather, the White House has worried that protracted political turmoil could provide an opening for additional influence by Tehran, whose nuclear ambitions are a concern to the U.S. and its allies in Europe and the Middle East.
So far, an administration official said, Iranian "aspirations far outpace their ability to project their influence into these places."

By disclosing intelligence about Iranian involvement, the U.S. appears to be trying to put Tehran on notice that it is under close surveillance in Washington. "We're keeping an eye on these activities," another Obama administration official said.

The U.S. disclosures also appear designed to help soothe anxious Arab and Israeli allies, who have privately complained that President Barack Obama, in his enthusiasm to embrace popular uprisings, is paying scant attention to how the revolts could play into the hands of their regional nemesis, Iran. By voicing concerns about Iran's activities, the U.S. appears to be trying to close ranks, at least in part, with Saudi and Bahraini leaders whose warnings about Tehran's influence in their internal affairs have long been played down in Washington.
Iranian diplomats didn't immediately respond to requests for comment.

Some U.S. officials have expressed surprise that Shiite-dominated Iran hasn't intervened more aggressively to support Mr. Assad and Shiites in Bahrain. Officials said they believed Iran has secretly promised more help to Mr. Assad if the protests intensify.
U.S. officials believe Iran's recent support for Mr. Assad reflects Tehran's concerns about losing a critical regional ally and military partner against Israel.

So far, officials said, Iran has begun transferring to Damascus equipment to help security forces put down protests. This includes providing Syrian authorities with equipment, advice and technical know-how to help curtail and monitor internal communications, including the email and online postings that opposition groups commonly use to organize their protests and report security excesses, officials said. Some deliveries have been made and others are believed to be in the works, they said.

Iran is also sharing "lessons learned" from its 2009 crackdown on protesters who demanded the removal of Iranian President Mahmoud Ahmadinejad, the officials said. "These guys know the best practice in this kind of situation—they've had lots of experience in this sphere," a U.S. defense official said of the Iranians.

"The Syrians don't want to see a Green Revolution in their country," the defense official added, referring to the protest movement in Iran. "The Iranians are ready to help."
Any aid to Mr. Assad could signal an escalation of sectarian proxy battles in the region, one the U.S. has sought to avert.

The Obama administration repeatedly pressed Sunni-ruled Saudi Arabia and Bahrain not to use force against largely Shiite protesters, according to U.S. officials, fearing that would provide Iran with an excuse to start meddling in Bahrain and elsewhere in the Middle East. Under Tehran's religious code, Supreme Leader Ayatollah Ali Khamenei has an obligation to protect the rights of Shiites world-wide. "We told them not to use force because it would provide Iran with an excuse," a senior U.S. official said. "They didn't listen."
Last month, Saudi Arabia sent troops into neighboring Bahrain to support the island kingdom's ruling al-Khalifa family against protesters.

The U.S. is concerned large-scale solidarity protests could break out in Iraq, whose Shiite majority has close religious ties to Bahrain's Shiites. That could complicate U.S. plans for withdrawing troops this year. The U.S. has long accused Iran of providing weapons, funding and training to anti-American militants in Iraq and to the Taliban in Afghanistan, as well as to Hezbollah in Lebanon.

The latest U.S. assessment is based on intelligence that includes intercepted communications among Iranian officials as well as between Iranian officials and Bahraini and Yemeni opposition figures. Military officials describe intercepted "chatter" in which Iranian officials have talked of the possibility of shipping cash, weapons or both to opposition elements in both countries.

A U.S. defense official said Iranian policy makers are seriously debating how much aid, if any, to provide to Bahrain's opposition. Another U.S. official said some intelligence indicated that Iran has made small-scale transfers of money and light weapons—"a few dozen guns, maybe less, definitely not more"—into Bahrain. Much of the intelligence suggests Iran and Hezbollah were focused now on using propaganda to assert influence among restive Shiites.
Other Iranian officials appear content to let Bahrain's leaders become more repressive, which the defense official said is "probably more effective at getting people riled up against the king" than anything Tehran could do.
The Bahraini and Yemeni governments have long claimed Iran is meddling in their internal affairs, an issue they know could alarm their U.S. counterparts.

U.S. intelligence agencies have long been skeptical of such claims. But last week, after talks in Riyadh, Defense Secretary Robert Gates said the U.S. has unspecified "evidence" of Iranian interference in Bahrain and elsewhere.
Shiite political leaders in Bahrain say that while they have cultural and religious connections to Iran, they aren't seeking help or guidance from abroad. They say accusations of Iranian involvement are designed to deflect their demands for democratic reforms and to justify Bahrain's widescale detention of suspected protest organizers, which the government has said it suspects of ties to Iran or its ally, Hezbollah.

"Bahraini Shia are very aware of how they're paying the price for Iran's growing power in the region," said Nabeel Rajab, president of the Bahrain Centre for Human Rights. "They know to keep their distance."
To keep a lid on tensions in Bahrain, which is home to the U.S. Navy's Fifth Fleet, the White House has encouraged protesters to negotiate with the king, rather than seek his ouster.
In Yemen, the U.S. has shifted from supporting President Ali Abdullah Saleh to backing talks aimed at easing him from power.

Last year, the U.S. picked up intelligence showing Iran had provided a small amount of support to Yemen's Houthi rebels, which have fought against Saudi and Yemeni forces, although officials say their knowledge is limited because of a lack of U.S. intelligence sources in the area.

The Houthis, who aren't part of the political opposition demanding Mr. Saleh's removal, have stayed quiet in their home region during the past two months of upheaval. The Houthis follow a minor offshoot of Shiism that isn't the same as the version practiced in Iran.

—Jay Solomon, Nathan Hodge, Margaret Coker and Bill Spindle contributed to this article