WASHINGTON—U.S. intelligence operatives and diplomats have stepped up their contacts with Syrian rebels in part to help organize their burgeoning military operations against President Bashar al-Assad´s forces, according to senior U.S. officials.

As part of the efforts, the Central Intelligence Agency and State Department—working with Saudi Arabia, Turkey, Qatar and other allies— are helping the opposition Free Syrian Army develop logistical routes for moving supplies into Syria and providing communications training.

U.S. officials also are considering sharing intelligence with the Free Syrian Army, or FSA, to allow the rebels to evade pro-Assad forces, which are believed to be getting intelligence, arms and communications support from Russia and Iran, the officials said. Iran it denies it is involved in Syria; Russia says the arms it sells Syria aren´t used in the crackdown.

Details of the deepening U.S. involvement comes as many international and local observers say Syria´s deadly 15-month conflict has reached new lows. On Wednesday, Secretary of State Hillary Clinton warned that "the situation is spiraling toward civil war."

The CIA´s heightened role is seen by some as a sign of growing U.S. seriousness about the military effort against the Assad government. U.S. officials also think that added pressure could force the regime to agree to a cease-fire.

The U.S. in many ways is acting in Syria through proxies, primarily Saudi Arabia, Qatar and the United Arab Emirates, say U.S. and Arab officials. Saudi Arabia is particularly fixated on overthrowing Mr. Assad, said Arab officials, viewing it as a way to settle scores with an arch foe and weaken its chief regional rival Iran.

Saudi Arabia and Qatar are providing the funds for arms, Arab officials and Syrian opposition leaders say. The Obama administration hasn´t agreed to arm the FSA, the U.S. officials stressed. Mrs. Clinton on Wednesday denied charges by Syria and others that the U.S. has armed the rebels.

The U.S.´s stepped-up links with the FSA are also part of an effort to gain a better understanding of the rebels´ capabilities and of the identities and allegiances of fighters spread in disparate groups across the country, the U.S. officials said. The U.S. officials remain wary of some rebels´ suspected ties to hard-line Islamists, including elements of al Qaeda. They acknowledge the FSA doesn´t represent all parts of the insurgency against the Assad regime.

But the administration hopes that their growing contacts will result in a more-organized fighting force that will shed more-troublesome associations.

"Some of [this communication] is dedicated to figuring out who these people are by talking to them," said a U.S. official briefed on Syria. "We´re not going to give out weapons and comms to people who can´t figure out how their chain of command works."

The U.S. operatives are drawing on their experience in Libya, and are conveying the message that the FSA needs to professionalize its ranks and better organize itself to receive further assistance, the official said.

"Recognizing that the phenomenon is not going to go away, we want it to have a command and control structure, and be responsive to civilian leadership at the local level," said a Western official who has worked with the Syrian opposition.

The U.S. has had diplomatic contacts with Syrian dissidents for more than six months. The CIA and State Department began stepping up contacts with the FSA around March, according to U.S. officials and Syrian opposition groups, due in part to the rising concerns about the presence of extremist groups, especially after twin bombings in Damascus that month.

In April, Mrs. Clinton said publicly that the State Department would begin providing communications equipment to the Syrian National Council, the umbrella group that brings together Syria´s main political opposition. Privately, American officials have acknowledged that much of this gear will end up with the FSA.

The State Department and CIA declined to comment.

U.S. defense officials and Syria analysts believe the FSA has grown into an increasingly sophisticated fighting force in recent months, after getting routed in the central Syrian city of Homs in February.

The flow of ammunition has increased to the FSA through Syria´s northern border with Turkey, they said. And the FSA´s internal command structure appears more organized and able to communicate to a sprawling mix of insurgent groups operating across the country.

The rebels have obtained increasingly lethal roadside bombs in recent months, as well as anti-tank rockets, say rebels and U.S. officials.

This week, Syrian rebels began to say publicly they are able to intercept government military communications. Rebel commanders also say new, secure communications between their ranks have allowed them to organize larger defections.

On Sunday, rebels said they had briefly overtaken an air-defense base that held advanced surface-to-air missiles and antiaircraft vehicles. The FSA´s operation to target the al-Ghanto missile base north of Homs is outlined in a series of videos posted on YouTube said to have been shot by rebels.

In the videos, commanders describe the orchestrated defection of soldiers and officers at the base, as well the swift regime attack that followed. It appeared to leave the area around the base on fire and destroy the arsenal of weapons and ammunition, said rebel officers involved in the alleged operation.

In one video, an officer says the missile base was completely destroyed in bombing by government helicopters after rebels there seized some weapons and ammunition. It isn´t clear what weaponry they may have made away with, but the reported incident illustrates a growing boldness among rebel fighters in attempting larger-scale operations.

"In the past two months, the rebels have shown renewed vigor.…They are pressing the regime on a lot of areas," said Jeffrey White, a former Defense Intelligence Agency analyst, now at the Washington Institute for Near East Policy. "The FSA is stretching the regime´s capabilities."

U.S. and Arab officials believe Mr. Assad is increasingly losing control of the Syrian countryside, even though he maintains power in cities like Damascus, Aleppo and Latakia. On Wednesday, the government said it regained control of Haffa, a rebellious city perched atop the mountainous Latakia coast, a government stronghold.

The president is also seen losing his ability to control supply routes connecting his forces to northern Syria and the coast.

"There´s a stalemate in which the government controls key major cities. But once you get off the main highway, the rebels basically own it," said Joseph Holliday, an analyst at Washington´s Institute for the Study of War.

The political resurgence of the exiled Syrian Muslim Brotherhood, the largest and only opposition group with experience in fighting the Assad regime, has also raised concerns in Washington that the loosely connected Syrian militias will pursue a bloody, chaotic and ultimately unsuccessful insurgency like the one the Brotherhood led in the 1980s.

To reassert influence, Syria´s Brotherhood, a large faction in the opposition Syrian National Council, has bypassed its parent coalition and created its own military bureau to funnel funds and arms to fighters in Homs and parts of Hama.

Some of these fighters, desperate for support, say they are halfheartedly pledging political allegiance to the Brotherhood—a short-term promise they say they intend to later betray. Already, rebel fighters say rival militias have fought each other—and other unidentified fighters—in hourslong battles in Homs and Idlib.

In recent weeks, rebel fighters have responded to international calls to better centralize command of the fight. They have created nine military councils at the level of Syria´s provinces led by appointed army defectors—rather than civilian fighters—that command smaller brigades. It is too soon to tell how such efforts will play out, with over 100 fighting groups spread across the country.

The growing instability in Syria is feeding a growing debate inside the Obama administration and allied governments about the potential need to intervene to stop the bloodletting inside Syria.

Washington is against taking military action in Syria without a formal mandate from the United Nations Security Council, something Russia and China have so far opposed. There is increasing talk of establishing buffer zones on Syria´s borders with Turkey and Jordan to protect civilians from Mr. Assad´s forces. Allies also have discussed providing greater security for U.N. monitors operating inside Syria.

These discussions come as senior American, Israeli and Arab officials have said in recent weeks that they are growing increasingly worried that Syria is degenerating into a failed state and that violence inside the country could spill into Iraq, Lebanon, Turkey and Jordan.

In a worst-case scenario, these officials said, the country could split into zones: with Mr. Assad and his closest allies—Iran and the Lebanese militant group Hezbollah—maintaining control of Syria´s northwest. Sunni extremists and Islamist fundamentalist groups, such as al Qaeda and the Muslim Brotherhood, could control other regions, while Kurdish groups would maintain their own areas.

Further feeding fears is the potential for Syria´s large stockpile of chemical weapons to fall into the hands of Hezbollah or al Qaeda, as Mr. Assad´s forces are no longer are able to secure arms depots. Such a threat, combined with the spreading violence, is causing some U.S. and allied officials to conclude that an intervention into Syria is inevitable at some juncture.

"Syria has the potential to be totally fragmented," said a senior Israeli official. "It has the potential to be the new model of Iraq. It will project into the whole region."

—Siobhan Gorman and Julian E. Barnes contributed to this article. (Copyright © Dow Jones & Company, Inc.) 06/14/12)