Syrian forces began combining chemicals that would be used to make deadly sarin gas for use in weapons to attack rebel and civilian populations, a U.S. official tells CNN.
The United States obtained intelligence over the weekend indicating this development, according to the official who had direct knowledge of the latest information.
The intelligence, the official said, came from multiple sources but declined to provide any more details about how the United states learned of it.
Sarin gas, the source said, could most readily be used to fill artillery shells.
Syrian State TV cited a Foreign Ministry official on Monday as saying the country would never use chemical weapons on its own people.
CNN reported on Sunday that U.S. intelligence is concerned about the Syrian government's intent regarding its vast chemical weapons stockpiles after what one senior American official described as "worrying signs" of activity in "the last few days."
The revelation that the regime of Syria's Bashar al-Assad is mixing chemicals underscores the reason for the increased concern.
"There are concerns the regime may be considering use of chemical weapons" the senior U.S. official told CNN on Sunday.
The official who spoke to CNN on Monday declined to be identified because of the sensitive nature of the information.
The blog Danger Room first reported the news on Monday, noting the activity is taking place in "central Syria."
Earlier in the day, Secretary of State Hillary Clinton warned the United States was prepared to act if Syria used chemical arms, saying that step would cross a red line previously drawn by President Barack Obama.
"I'm not going to telegraph in any specifics what we would do in the event of credible evidence that the Assad regime has resorted to using chemical weapons against their own people. But suffice it to say, we are certainly planning to take action if that eventuality were to occur," Clinton told reporters.
"There is no doubt that there is a line between even the horrors that they have already inflicted on the Syrian people and moving to what would be an internationally condemned step of utilizing their chemical weapons."
Clinton met with the Czech foreign minister and discussed the concerns, she said after the meeting. Clinton said the Czech Republic has extensive expertise with chemical and biological weapons.
"They have already been consulting about what can be and should be done, both at this time and post the inevitable fall of the Assad regime," Clinton said.
Syria is known to store its chemical stockpile, in many instances, separately from the artillery shells, rockets or missiles that would deliver those chemical weapons in an attack.
As rebel fighters continue to make some gains, capturing military assets and territory, U.S. and Middle Eastern intelligence services have been watching for some months for any signs the Syrians would be loading up those weapons with chemical agents.
In recent months, the Obama administration has acknowledged at least two instances of Syrian chemical weapons stockpiles being moved but has said it believes those were efforts to put the material in more secure locations as fighting increased.
"We don't know if there is actual intent," the U.S. official said about the latest observed activities. "This is worrisome. This is a step beyond moving them around."