The United States has intelligence that Syria has moved chemical weapons at some sites for security reasons, Defense Secretary Leon Panetta said.

There has been "limited movement" at Syria's major chemical storage sites, Panetta said at the Pentagon in response to questions from CNN's Barbara Starr.

But the United States believes the stashes remain secured by the Syrian military, adding the sites are being monitored by the U.S. and other countries.

"We've continued to monitor that. We are working with countries in the region to ensure that we have the best information possible with regards to the sites and how they are being secured," Panetta said.

Though Panetta said he was unsure of where these movements had occurred, he said the steps were taken "for the Syrians to better secure the chemicals."

Concern first arose in July when satellite surveillance and communications intercepts revealed some initial stockpile movements.

Panetta said he could not say whether Syrian rebels have gained access to any chemical weapons.

"With regards to the movement of this and whether or not they've been able to locate some of it, we just don't know," Panetta said.

The existence of chemical weapons in Syria and the danger they pose in the war-wracked country has heightened a sense of urgency among world powers working to end the civil war.

President Barack Obama said in August that Syrian President Bashar al-Assad should heed U.S. warnings to neither use nor move chemical or biological weapons, lest he risk crossing a "red line" and provoke an American military response.

"We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized. That would change my calculus. That would change my equation," Obama said.

When asked whether Syria has now violated that "red line" set by Obama, a senior administration official told CNN Friday: "(Panetta) says 'limited movement' for security reasons. The president was talking about proliferation of (weapons of mass destruction) or giving it to other bad actors."