Washington: The Syrian military's movement of chemical weapons in recent days has prompted the US and several allies to repeat their warning to the President, Bashar al-Assad, that he would be "held accountable" if his forces used the weapons against the rebels fighting his government.
The warnings, which one European official said were "deliberately vague to keep Assad guessing," were conveyed through Russia and other intermediaries.
Obama warns Syria over chemical weapons
US officials say intelligence has detected Syrian movement of chemical weapons components in recent days.
What the Syrian forces intend to do with the weapons remains murky, according to officials who have seen the intelligence from Syria.
One US official provided the most specific description yet of what has been detected, saying "the activity we are seeing suggests some potential chemical weapon preparation" which goes beyond the mere movement of stockpiles among Syria's several dozen known sites.
But the official declined to be more specific about what those preparations entailed.
During the weekend, the activity in Syria prompted emergency communications among Western allies, which have been developing contingency plans in case they decide to intervene to neutralise the weapons, a task the Pentagon estimates would require more than 75,000 troops.
But there were no signs that preparations for any such effort were about to begin.
So far, the US President, Barack Obama, has been cautious about intervening in Syria, declining to arm the opposition groups directly, or even to formally recognise a newly formed coalition of opposition forces that the US helped create.
But at a news conference in August, Mr Obama said any evidence that Mr Assad was moving the weapons in a threatening way or using them would be "a red line" that could prompt direct US intervention.
"That would change my calculus," he said. "That would change my equation."
US officials would not say during the weekend whether the activity they were seeing edged towards the limit set by Mr Obama.
"These are desperate times for Assad, and this may simply be another sign of desperation," said one senior US diplomat involved in the effort to dissuade Mr Assad's forces from using the chemical weapons.
A senior Israeli official said it was possible the movement of the weapons – and the apparent preparations to use them – could be a bluff, intended as a warning to the West when the North Atlantic Treaty Organisation and the US were debating greater support to opposition groups.
"It's very hard to read Assad," one Israeli official said. "We are seeing a kind of action that we've never seen before." He declined to elaborate.
The White House would not comment on the intelligence reports but a senior administration official 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."
Another senior US official refused to give more detail about what the Syrians were doing, saying only, "There's a sufficient degree of concern to warrant even closer monitoring. We're concerned about it, to be sure."
Several months ago, the US military quietly sent a taskforce 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 anti-missile system, in part as protection against Syrian missiles that might come into Turkish territory.
To make the case for the emplacements, the Turks have raised the possibility that chemical weapons could be used in the warheads.
This is not the first time activity at stockpile sites has been detected. On September 28 the US Defence Secretary, Leon Panetta, said 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," Mr Panetta said then.
But the recent 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 its vast stockpile, the US and its allies have increased electronic eavesdropping and other surveillance activities of the sites. A senior defence official said no US troops had been put on heightened alert but the Pentagon was prepared to do so, if necessary.
"From the beginning of this crisis, we have conducted prudent military planning so that we are prepared for all contingencies," the official said.
The New York Times