Western intelligence agencies observed Syrian units making advanced preparations for the potential use of chemical weapons, including loading trucks with ready-to-use bombs and shells, prompting President Obama last week to warn Syria against using the banned munitions, according to Western and Middle Eastern officials.

Soldiers at one Syrian base were monitored mixing precursors for chemical weapons and taking other steps to ready the lethal munitions for battlefield use, the officials said. It was the first hard evidence that Syria was moving toward possible activation of its vast arsenal of chemical weapons, which includes nerve gas and other poisons.

Surveillance photos confirmed that at least one army unit began loading special military vehicles that transport bombs and artillery shells carrying chemical warheads, according to the officials. The moves followed specific orders to elite troops to begin preparations for the use of the weapons against advancing rebel fighters, the officials said.

Two Western officials briefed on the intelligence findings said that the Syrian government forces stopped the preparations late last week and that there was no evidence that activated chemical weapons were loaded onto aircraft or deployed to the battlefront.

The officials spoke on the condition of anonymity because of the sensitive nature of the intelligence. The Obama administration and the CIA declined to answer questions about the episode. Defense Secretary Leon E. Panetta said this week that the threat had eased, though it had not been eliminated.

Timeline: Major events in Syria’s tumultuous uprising that began in March 2011.

Intelligence analysts said the orders to prepare the weapons were issued about two weeks ago. They said it was not clear whether the decision came from senior Syrian leaders, possibly including President Bashar al-Assad, or from a field commander acting on his own, the officials said.

Since concerns surfaced in the summer that Syria was moving chemical weapons among several sites across the country, officials in Damascus have repeatedly pledged not to use the banned munitions. After the warnings last week from Obama and other foreign leaders, the Syrian Foreign Ministry repeated that it would not use chemical weapons against the rebel forces.

Still, the discovery that steps had been taken to activate weapons at at least one military base alarmed intelligence officials, because of fears that a single commander could unleash the deadly poisons without orders from higher up the chain of command.

Danger persists

The latest disclosures, which provide more detail about the weapons threat than was previously known, came amid reports that Syrian troops have launched short-range, Scud-type missiles against rebel positions in recent days in an escalation of the nearly 21-month-old conflict. Several types of surface-to-surface missiles in Syria’s arsenal are capable of carrying chemical-­weapons warheads. There have been no reports that the missiles launched contained chemical weapons.

Syria is known to possess one of the world’s largest arsenals of chemical weapons, including stocks of the highly lethal nerve agents sarin and VX. The chemicals can be loaded into artillery shells, aerial bombs or missile warheads for use against troop positions or civilian targets.

It is unclear whether the warnings from Obama and others factored into the Syrian government’s decision to put the limited chemical weapons preparations on hold. Syrian commanders have shown no hesitancy in using other lethal weapons — including cluster bombs and incendiary devices — to slow a rebel offensive that is pressing ever closer to the capital, the officials said.

“If the situation becomes more desperate, there’s no predicting what will happen,” said a Western diplomat whose government tracked the developments as they began unfolding nearly two weeks ago.

Interactive: Recent events in Syria

On Thursday, NATO Secretary General Anders Fogh Rasmussen said Assad’s government appears to be approaching collapse, and Russia acknowledged that Syrian rebels were making progress in their effort to oust Assad, a Moscow ally.

Although Assad is aware of the dire consequences of using chemical weapons, individual commanders could take matters into their own hands if their positions are being overrun, said a Middle Eastern intelligence official briefed on the latest intelligence findings.

“Once you’ve used the weapons, you know the world is coming after you,” the official said. “But if you’re a general and you think you’re not going to survive this, you might not care.”

Panetta confirmed Tuesday that the threat appeared to have eased after Obama publicly warned Assad of “consequences” if he used chemical weapons, which are banned under international treaties.

“We haven’t seen anything new indicating any aggressive steps to move forward in that way,” Panetta told reporters during a visit to Kuwait. Referring to Obama’s warning to Assad, the defense secretary said, “I like to believe he’s got the message.”

A game-changer

Syria began amassing its chemical weapons arsenal in the 1970s and 1980s as a counterbalance against a superior-armed Israel, its presumed foe in a future conflict. Military and intelligence analysts are divided over whether Assad would use the munitions against his own people, an act that would earn him global condemnation and perhaps trigger an attack by Western forces.

Chemical weapons such as sarin are designed for use against massed concentrations of troops and are not regarded as particularly effective against insurgencies in close-combat situations. Yet, a large-scale use of such munitions could devastate the rebels by causing panic and, potentially, thousands of casualties.

“It could change the game in important ways,” said Jeffrey White, a military analyst formerly with the Pentagon’s Defense Intelligence Agency. “Any use of gas would have a terrific psychological impact and cause all kinds of panic.”

Clouds of poison gas also could kill government supporters and troops if a subtle wind shift blows the chemicals toward government-held lines, White said. “It doesn’t spread for miles,” he said, “but you have to think that any group affected by it will be highly traumatized, and word will spread quickly.”

Depending on the type and quantity of weapons used, the attackers could deny access to large swaths of territory because of the long-lasting effects of the poisons. Nerve agents such as sarin are so deadly that a small drop on the skin can kill a person. Even the task of removing or treating victims of a chemical attack can prove deadly for rescue workers and physicians, weapons experts say.

Although experts generally agree that any use of chemical weapons would seal Assad’s doom, some analysts say the embattled Syrian president may not be convinced that the international community would strike back.

“We haven’t seen a lot of willingness in this conflict to confront Assad on any level,” said Bilal Saab, director of the Institute for Near East and Gulf Military Analysis-North America, a think tank based in the United Arab Emirates. “Assad needs to be told, not just in words but by actions on the ground, that there will be consequences.”