A Western diplomat briefed on the findings by the inspectors from the global Organization for the Prohibition of Chemical Weapons said that while there was no clear evidence of new use or production of forbidden chemicals, “the strong suspicion is they are retaining stockpiles which are being held back.”

“This, and the open defiance in using prohibited chlorine bombs, is indicative of bad faith from the beginning,” the diplomat said.

A violation of the deal struck to eliminate Syria’s chemical arsenal would undermine President Obama’s single claim to policy success in Syria, where four years of war has displaced nearly half the country, killed more than 200,000 people and shown no sign of abating. It could also embarrass Russia, Mr. Assad’s most powerful military and diplomatic patron.

Already there was mounting evidence that Mr. Assad’s forces had violated the terms of the international treaty banning use of chemical weapons — and signed as part of the deal facilitated by Washington and Moscow — by dropping jerry-built chlorine bombs on insurgent-held areas. While the Syrian government has denied using chlorine or any chemical as a weapon, only the government has access to the helicopters that witnesses say are being used, and rescue workers say the pace of such attacks has increased in some areas.

Now, inspectors are trying to determine the significance of the new findings of small amounts of banned agents, which do not necessarily indicate a lingering weapons program and were described by some diplomats as being perhaps less troublesome than the charges that the Syrian government is dropping barrel bombs loaded with chlorine. The inspectors, whose last tour ended on April 3, are expected to seek answers from Syrian officials during a visit beginning Sunday.