Engineers working for the Assad regime in Syria have begun combining the two chemical precursors needed to weaponize sarin gas, an American official with knowledge of the situation tells Danger Room. International observers are now more worried than they've even been that the Damascus government could use its nerve agent stockpile to slaughter its own people.

The U.S. doesn't know why the Syrian military made the move, which began in the middle of last week and is taking place in central Syria. Nor are they sure why the Assad government is transferring some weapons to different locations within the country, as the New York Times reported on Monday.

All that's certain is that the arms have now been prepped to be used, should Assad order it.

"Physically, they've gotten to the point where the can load it up on a plane and drop it," the official adds.

Sarin gas has two main chemical components —  isopropanol, popularly known as rubbing alcohol, and methylphosphonyl difluoride. The Assad government has more than 500 metric tons of these precursors, which it ordinarily stores separately, in so-called "binary" form, in order to prevent an accidental release of nerve gas.

Last week, that changed. The Syrian military began combining some of the binaries. "They didn't do it on the whole arsenal, just a modest quantity," the official says. "We're not sure what's the intent."

Back in July, the Assad regime publicly warned that it might use its chemical weapons to stop "external" forces from interfering in Syria's bloody civil war. The announcement sparked a panic in the intelligence services of the U.S. and its allies, which stepped up their efforts to block shipments of precursors for those weapons from entering the country.

"This is a more serious moment than July," according to the official.

At the Pentagon, chief spokesman George Little said that "any consideration of the use of chemical weapons by the use of the Syrian regime would be unacceptable." In Prague, Secretary of State Hillary Clinton called the use of those weapons "a red line" that would prompt a U.S. response, which Little declined to elaborate upon.

"The Syrian regime must maintain security over their chemical weapons stockpiles and must not use chemical weapons against their own people," Little said.

It's unclear whether the chemical weapons movements is preparation for an outright offensive. But Mideastern nations and international groups are taking precautions, because of the dangers both conventional and not. The U.N. is pulling non-essential staff out of Damascus. Egypt ordered a commercial plane to Damascus to turn back around, and Israeli officials have quietly mooted a plan to the Jordanians for taking out the chemical sites, the Atlantic reported. Pentagon officials have suggested securing those sites would require some 75,000 troops — an indication of the Pentagon's reluctance to deepen its involvement in Syria.

Assad's position appears to have weakened recently. Fighting around the Syrian capital of Damascus has intensified, as rebel troops captured a half-dozen bases around the city. Damascus' airport, a major logistics hub, is under threat, and the regime was forced to bring in "troop reinforcements to secure the route to the airport," reported the Los Angeles Times. The rebel gains have been the most dramatic in the north and east. Over the course of last month, rebel forces overran more towns near the Turkish border and seized a major military base near the besieged city of Aleppo, which netted an arsenal of heavy weapons including tanks, howitzers and armored personnel vehicles. Assad has continued to retaliate, with more reports of cluster bomb attacks filtering out of Syria, and another round of air attacks on rebel-controlled villages.

Even Assad's ability to strike the rebels from the air is under challenge. In two videos posted online in recent weeks, a Syrian air force Mi-8 helicopter operating near Aleppo was seen being shot down by a missile. In another set of videos that appeared online prior to the attack, the rebels have what appears to be SA-7 man-portable anti-aircraft weapons, or MANPADS. And those might not be the only ones the rebels have. A series of photos on rebel Facebook pages, collected by blogger Eliot Higgins at the Brown Moses blog -  a go-to source for images of the Syrian conflict — rebels associated with extremist group Ansar al-Islam are shown posing with what appears to be incomplete SA-16 and SA-24 anti-aircraft missile tubes. Those Russian-made MANPADS are some of the most advanced on the market and pose a major proliferation risk.

On Thursday, Syria abruptly became disconnected from the internet, likely after the regime disabled the four cables that provide Syria with connectivity. The rebels use the internet not only to document regime atrocities but to disseminate training tactics and to spread their propaganda,. Yet the regime also relies on the internet: it's tried to hijack rebel hardware by spreading spyware in the form of fake security software. As Danger Room predicted last week, the outage ended quickly, as online monitor Renesys confirmed a "largely complete restoration of the Syrian Internet" by Saturday.

The U.S. official doesn't believe the internet blackout was related to the combination of the chemical weapon binaries. And at the Pentagon, Defense Department spokesman Little said the online outage didn't make a difference for the U.S. understanding of Assad's dangerous weapons. "The U.S. government has good visibility into the chemical weapons program and we continue to monitor it," Little said.

One U.S. official worried to CNN that "this puts us into the contingency of potential U.S. action." But it's far from clear what a U.S. response might look like. The Pentagon has suggested that securing Syria's chemical stockpiles will be a massive undertaking, requiring some 75,000 troops, while the Afghanistan war drags on. NATO will decide a early as Tuesday whether to station Patriot missile batteries in Turkey, but that defensive measure is a far cry from a decision to seize Syria's chemical weapons. Until recently, that seemed like an extremely remote possibility. But that was before Assad's engineers began their poisonous combinations.

- additional reporting by Robert Beckhusen