Adding to American concerns, Western intelligence officials say they are picking up new signs of activity at sites in Syria that are used to store chemical weapons. The officials are uncertain whether Syrian forces might be preparing to use the weapons in a last-ditch effort to save the government, or simply sending a warning to the West about the implications of providing more help to the Syrian rebels.

“It’s in some ways similar to what they’ve done before,” a senior American official said, speaking on the condition of anonymity to discuss intelligence matters. “But they’re doing some things that suggest they intend to use the weapons. It’s not just moving stuff around. These are different kind of activities.”

The official said, however, that the Syrians had not carried out the most blatant steps toward using the chemical weapons, such as preparing them to be fired by artillery batteries or loaded in bombs to be dropped from warplanes.

Regarding the arms shipments, Secretary of State Hillary Rodham Clinton secured a commitment from Iraq’s foreign minister in September that Iraq would inspect flights from Iran to Syria. But the Iraqis have inspected only two, most recently on Oct. 27. No weapons were found, but one of the two planes that landed in Iraq for inspection was on its way back to Iran after delivering its cargo in Syria.

Adding to the United States’ frustrations, Iran appears to have been tipped off by Iraqi officials as to when inspections would be conducted, American officials say, citing classified reports by American intelligence analysts.