WASHINGTON » In the last days of November, Israel’s top military commanders called the Pentagon to discuss troubling intelligence that was showing up on satellite imagery: Syrian troops appeared to be mixing chemicals at two storage sites, probably the deadly nerve gas sarin, and filling dozens of 500-pound bombs that could be loaded on airplanes.

Within hours President Barack Obama was notified, and the alarm grew over the weekend, as the munitions were loaded onto vehicles near Syrian air bases. In briefings, administration officials were told that if Syria’s increasingly desperate president, Bashar Assad, ordered the weapons to be used, they could be airborne in less than two hours — too fast for the United States to act, in all likelihood.

What followed next, officials said, was a remarkable show of international cooperation over a civil war in which the U.S., Arab states, Russia and China have almost never agreed on a common course of action.

The combination of a public warning by Obama and more sharply worded private messages sent to the Syrian leader and his military commanders through Russia and others, including Iraq, Turkey and possibly Jordan, stopped the chemical mixing and the bomb preparation. A week later Defense Secretary Leon E. Panetta said the worst fears were over — for the time being.

But concern remains that Assad could now use the weapons produced that week at any moment. U.S. and European officials say that while crisis was averted in that week from late November to early December, they are by no means resting easy.

"I think the Russians understood this is the one thing that could get us to intervene in the war," one senior defense official said last week. "What Assad understood, and whether that understanding changes if he gets cornered in the next few months, that’s anyone’s guess."

While chemical weapons are technically considered a "weapon of mass destruction" — along with biological and nuclear weapons — in fact they are hard to use and hard to deliver. Whether an attack is effective can depend on the winds and the terrain. Sometimes attacks are hard to detect, even after the fact. Syrian forces could employ them in a village or a neighborhood, some officials say, and it would take time for the outside world to know.

But the scare a month ago has renewed debate about whether the West should help the Syrian opposition destroy Assad’s air force, which he would need to deliver those 500-pound bombs. The chemical munitions are still in storage areas that are near or on Syrian air bases, ready for deployment on short notice, officials said.

The Obama administration and other governments have said little in public about the chemical weapons movements, in part because of concern about compromising sources of intelligence about the activities of Assad’s forces. This account is based on interviews with more than half a dozen military, intelligence and diplomatic officials, all of whom spoke on the condition of anonymity because of the intelligence matters involved.

The head of Germany’s foreign intelligence service, the BND, warned in a confidential assessment last month that the weapons could now be deployed four to six hours after orders were issued, and that Assad had a special adviser at his side who oversaw control of the weapons, the German newsmagazine Der Spiegel reported. Some U.S. and other allied officials, however, said in interviews that the sarin-laden bombs could be loaded on planes and airborne in less than two hours.

"Let’s just say right now, it would be a relatively easy thing to load this quickly onto aircraft," said one Western diplomat.

How the United States and Israel, along with Arab states, would respond remains a mystery. U.S. and allied officials have talked vaguely of having developed "contingency plans" in case they decided to intervene in an effort to neutralize the chemical weapons, a task that the Pentagon estimates would require upward of 75,000 troops. But there have been no evident signs of preparations for any such effort.

The U.S. military has quietly sent a task force of more than 150 planners and other specialists to Jordan to help the armed forces there, among other things, to prepare for the possibility that Syria will lose control of its chemical weapons.

Israel’s prime minister, Benjamin Netanyahu, was reported to have traveled to Jordan in recent weeks, and the Israeli news media have said the topic was how to deal with Syrian weapons if it appeared they could be transferred to Lebanon, where Hezbollah could lob them over the border to Israel. But the plans, to the extent they exist, remain secret.

U.S., Israeli and other allied officials remained fixed on this potential crisis, especially as the opposition appears to have gained more momentum, seizing several Syrian military bases and the weapons stored there, and have been closing in on Damascus, the Syrian capital.

In response, Syria has reached deeper into its conventional arsenal, including firing Scud ballistic missiles at rebel positions near Aleppo.

Over the past week a new concern emerged: Syrian forces began shooting new, accurate short-range missiles, believed to have been manufactured in Iran. None had chemical warheads. But their use showed that the Syrian military was now deploying a more accurate weapon than the notoriously inaccurate Scud missiles they have used in previous attacks.

As the fighting has escalated, U.S. and other allied officials have said that government troops have moved some of the chemical stockpiles to safer locations, a consolidation that, if it continues, could actually help Western forces should they have to enter Syria to seize control of the munitions or destroy them.

Syria’s chemical weapons are under the control of a secretive Syrian air force organization called Unit 450, a highly vetted outfit that is deemed one of the most loyal to the Assad government given the importance of the weapons in its custody.

U.S. officials said that some of the back-channel messages in recent weeks were directed at the commanders of this unit, warning them — as Obama warned Assad on Dec. 3 — that they would be held personally responsible if the government used its chemical weapons.

Asked about these communications and whether they have been successful, a U.S. intelligence official said only, "The topic is extremely sensitive and public discussion, even on background, will be problematic."

Allied officials say whatever safeguards the Syrian government has taken, there remains great concern that the weapons could fall into the hands of Islamist extremists fighting the government, or the militant group Hezbollah, which has established small training camps near some of the storage sites.

"Militants who got their hands on such munitions would find it difficult to deploy them effectively without the associated aircraft, artillery or rocket launcher systems," said Jeremy Binnie, a terrorism and insurgency specialist at IHS Jane’s Defense Weekly. "That said, Hezbollah would probably be able to deploy them effectively against Israel with a bit of help."

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