The United States believes Syria has moved "some" chemical weapons in recent days, a U.S. official said.
The stockpile is believed to be is under the control of regime forces, the source said.
The official, who would only speak anonymously because the source was speaking about intelligence matters, said the reason behind the movement is unclear. The source could not say what types of chemical stockpiles were involved or how much was moved.
The United States has had satellite surveillance of Syria's key weapons sites since the unrest began.
But based on conversations CNN has had with other sources knowledgeable about this matter, it's also likely some of the information came from communications intercepts.
Possible reasons for the move could range from wanting to better protect the material from the fighting of the spreading revolt in Syria or, more ominously, to use it against the population, the sources said.
The Wall Street Journal was first to report the suspicion.
The Pentagon has refused all comment. State Department spokeswoman Victoria Nuland said Syria's government needs to ensure protection of its weapons stash.
"We have repeatedly made it clear that the Syrian government has a responsibility to safeguard its stockpiles of chemical weapons, and that the international community will hold accountable any Syrian officials who fail to meet that obligation," Nuland said.
A former CIA director, Michael Hayden, said it is more likely that the weapons are being moved for security reasons. But Hayden, who said he has no knowledge of what U.S. officials are seeing, said it is always concerning when such materials are being moved.
"We were always concerned when states we believe had or might have weapons of mass destruction, when those weapons were being moved, when they were in transit," he told CNN, "because there is no way you can make the security of those devices as great when they are on the move as they are in firm, fixed positions, where you have a heavy security routine."
Hayden said it is hard to believe al-Assad's government would use the weapons against its own people.
"Right now they are on the outer edges of being an international outlaw, beyond the edges. And the use of chemical weapons would seem to me to make any opposition to a more active intervention in Syria impossible, even for the Russians, even for the Chinese," Hayden said. "I'm far more concerned about loss of control of the weapons and what happens when the chaos that seems to be affecting larger society might touch upon some of these weapon stockpiles."
Key senators who are pressing for U.S. military involvement to help the opposition were quick to demand more information about the Wall Street Journal story.
"First and foremost it is essential to determine whether this report is accurate, and if Assad is in fact moving chemical weapons, where and why they are being moved. We urge the administration to brief Congress on what is known as soon as possible," read a statement from Sens. John McCain, R-Arizona, Joseph Lieberman, I-Connecticut, and Lindsey Graham, R-South Carolina.
"If Assad is transferring chemical weapons from secure sites to the battlefield, it significantly raises the risks that they will be used or that control over these weapons could be compromised. These are unacceptable risks for the United States and the entire international community, and they would threaten our vital national security interests."
The United States believes the facilities are guarded by some of the most elite Alawite troops loyal to al-Assad.
But a senior U.S. official told CNN in June, when the opposition forces appeared to be gaining strength in some areas, the United States, Jordan and others were concerned that if the amount of area controlled by al-Assad shrinks, some of those critical facilities could become open to attacks, pilfering or efforts by terrorist groups to buy material.
"This is getting a fair amount of attention," another U.S. official told CNN in June. Also discussed with Jordanian forces was the possible need for U.S. chemical and biological weapon-detecting equipment, the official said.
The overall assessment by the United States is that in the event some action has to be taken to secure Syrian chemical or biological weapon facilities, troops from some country would have to enter Syria in a matter of hours.
The U.S. military has calculated it could take more than 75,000 ground troops to secure Syria's chemical warfare facilities if they were at risk of being looted or left unguarded, CNN reported earlier this year.
The conclusion came from a military analysis of options for Syria that the Department of Defense is preparing for president should he request it, according to a senior U.S. official.
Securing Syria's chemical sites would be "extraordinarily difficult" given the scope of the problem, a Department of Defense official told CNN in February. Both officials would speak only on the condition their names not be used because they were talking about military planning.
The U.S. military believes there are 50 chemical weapon and production sites spread across the country with additional storage sites and research centers as well. The cities of Hama, Homs and al Safira, and the port city of Latakia are all believed to house production facilities.
Syria is believed to have one of the most advanced chemical warfare capabilities in the region, with the ability to develop and produce agents such as mustard gas, sarin and possibly the VX nerve agent, according to information collected by the Nuclear Threat Initiative, a non-profit group that seeks to reduce the risk of use of chemical, biological and nuclear weapons.