The White House refused to comment on the intelligence reports, which have been shared with senior members of Congress. But a senior administration official, asked about the concerns, issued a new warning to the Syrians.

“The president has made it clear that the use of chemical weapons in Syria would cross a red line for the United States,” the official said. “We consistently monitor developments related to Syria’s stockpiles of chemical weapons, and are in regular contact with international partners who share our concern.

“The Assad regime must know that the world is watching, and that they will be held accountable by the United States and the international community if they use chemical weapons or fail to meet their obligations to secure them.”

Representative Mike Rogers, a Michigan Republican who heads the House Intelligence Committee, declined to comment on the new intelligence reports but said in a statement late Sunday: “We are not doing enough to prepare for the collapse of the Assad regime, and the dangerous vacuum it will create. Use of chemical weapons by the Assad regime would be an extremely serious escalation that would demand decisive action from the rest of the world."

Several months ago, the United States military quietly sent a task force of more than 150 planners and other specialists to Jordan to help the armed forces there to, among other things, prepare for the possibility that Syria would lose control of its chemical weapons. Turkey has asked NATO for two batteries of the Patriot antimissile system, in part as protection against Syrian missiles that might come into Turkish territory. In making their case, the Turks have raised the possibilities that chemical weapons could be used in the warheads.

This is not the first time activity at stockpile sites has been detected. Defense Secretary Leon E. Panetta said on Sept. 28 that there had been “some movement” of Syria’s chemical weapons stockpiles to put them in more secure locations. “While there’s been some limited movement, again, the major sites still remain in place, still remain secure,” he said at the time.

But the new activity appears to be of a different nature, and officials are no longer willing to say that all the sites remain secure. “We’re worried about what the military is doing,” one official said, “but we’re also worried about some of the opposition groups,” including some linked to Hezbollah, which has set up camps near some of the chemical weapons depots.

Since the crisis began in Syria and concern has been focused on the country’s vast stockpile, the United States and its allies have increased electronic eavesdropping and other surveillance activities of the sites. A senior defense official said that no United States troops had been put on heightened alert in response to the activity, although the Pentagon was prepared to do so, if necessary.