For months, senior members of the Obama administration have proclaimed that Syrian President Bashar al-Assad’s regime is on the brink of collapse, and for months, Assad’s government has withstood onslaughts by the opposition, economic sanctions and international pleas for a resolution to the crisis.

The bombing that killed at least three senior Syrian security officials on Wednesday, however, proved to be the clearest indication yet that Assad’s government is vulnerable, providing reason both for hope and anxiety among U.S. officials and analysts as they monitor the fallout.

In Washington, Defense Secretary Leon E. Panetta told reporters that the situation in Syria is “rapidly spinning out of control,” and President Obama cautioned Russian President Vladimir Putin in a morning phone call that his country’s continued support for Assad would put it “on the wrong side of history.”

But the diplomatic stalemate was unbroken. A critical vote by the U.N. Security Council on whether to impose tougher sanctions on Syria was postponed, and Russian Foreign Minister Sergei Lavrov said Assad would not be pushed out.

Whether the Damascus bombing becomes a watershed moment or a temporary — if significant — setback for Assad’s regime depends on Syria’s ability to preserve the cohesion and confidence of the military over the next few days, U.S. analysts said.

“The attack demonstrates that the rebels have deeply infiltrated the regime’s security apparatus, and that some portion of the regime’s security structure can be used against itself,” said Army Lt. Col. Joel Rayburn, an instructor at the Pentagon’s National Defense University.

Early indicators of which direction the situation is headed are likely to be whether the elite military units in the capital hold together and whether the Syrian generals need to shift brigades from outlying areas to Damascus.

Members of Assad’s inner circle may stick together because they have no viable alternatives beyond standing and fighting the rebel onslaught. But Syrian military units, arrayed in defense of the capital, could start to unravel over the next several days.

Any move by the Assad regime to shift battalions into the capital from other parts of Syria could embolden rebels in outlying areas and be a sign that it is on the verge of falling.

“If the military holds, the regime holds,” said Mark Kimmitt, the assistant secretary of state for political and military affairs in the George W. Bush administration. “When you start seeing the defection of elite units, that is truly the beginning of the endgame.”

In addition, analysts said it would be significant if the brazen attack increases the rate of military defections.

The conflict in Syria is “a war about defection,” centered on calculations inside the country of “when do you abandon this regime,” said Daniel Byman, a professor at Georgetown University and a former CIA analyst.

U.S. intelligence officials said that analysts have reached no conclusions about who is behind the bombing, even as two groups stepped forward to assert responsibility.

The first was an organization calling itself the Sahaba Brigades, described as an elite unit within the Free Syrian Army. In a video posted on YouTube, one of the group’s commanders, 1st Lt. Ahmed Muhammad Taqa, said that the group had conducted two months of surveillance on the “crisis control cell” and that the attack was carried out by one of the “heroes inside the place,” according to a translation by the SITE Intelligence Group, which monitors militant postings on the Internet.

In a reflection of the fractured nature of the insurgency, which is made up of Islamists and more secular fighters, a second group, calling itself “Brigade of Islam,” posted a claim on Facebook saying it had carried out the attack, according to SITE.

U.S. intelligence analysts played down the possibility that al-Qaeda had a role in the attack. One official said the radical jihadist group appeared to be “disconnected from the rest of the opposition” and “not representative of the wider struggle to remove the Assad regime from power.”

Indeed, one of the senior Syrian officials killed in Wednesday’s bombing, Gen. Assef Shawkat, was accused by U.S. officials of playing a central role in the Iraq war by shuttling foreign fighters allied with al-Qaeda into the country to battle U.S. forces.

The diverse and disorganized nature of the insurgency in Syria suggests that even if Assad’s military collapsed and the regime fell, the fighting in the country would not necessarily cease. Some officials expressed concern about what would happen to Syria’s stockpile of chemical weapons if the country descends into chaos and warned that, absent a negotiated settlement, the bloodshed in Syria could rise.

“If the regime begins to unravel, there is a risk of much more uncertainty and violence,” said Vali Nasr, the dean of the Johns Hopkins School of Advanced International Studies. “The fighting in Syria was never just about Assad.”