The months-long effort to dismantle Syria’s chemical weapons program has ground to a halt because Syria is holding on to 27 tons of sarin precursor chemicals as leverage in a dispute with the international community over the future of facilities used to store the deadly agents, according to U.S. officials.

Having turned over all but an estimated 8 percent of its chemical arsenal to the Organisation for the Prohibition of Chemical Weapons, Damascus missed a deadline Sunday to relinquish the remnants of its arsenal, which are stored in 16 containers in Damascus, U.S. officials said.

The OPCW is insisting that a network of tunnels and buildings that were used to store the weapons must be destroyed. The government of Syrian President Bashar al-Assad has argued that the facilities should be repurposed.

“They’re just stalling for time to hold on to some of these facilities,” said a U.S. official familiar with the matter who would discuss the issue only on the condition of anonymity. The official said he expects that Syria will ultimately give up the material.

Still, the relative success of an agreement Washington struck with Moscow last fall after a chemical weapons attack in a Damascus suburb has been dampened by reports that the Syrian government has begun using chlorine in rudimentary bombs dropped from aircraft in residential areas.

The use of the widely available industrial chemical in munitions known as barrel bombs would constitute a violation of the Chemical Weapons Convention, which Damascus joined last fall under the threat of U.S. cruise missile strikes. The OPCW, an organization created to enforce the terms of the convention, announced this week that it plans to dispatch a team to investigate reports of an April 11 attack using chlorine-laced bombs in Kafr Zita, an opposition-controlled village in northwestern Syria. There have been a handful of other reported instances with attacks involving chlorine.

“These reports are too serious to be ignored,” Robert P. Mikulak, the U.S. representative to the executive council of the OPCW, said Tuesday in public remarks in The Hague, where the group is based. “The United States considers them to be a matter of serious concern requiring an immediate international effort to determine what has happened.”

A senior U.S. intelligence official said U.S. intelligence agencies have little doubt that the attack was carried out by the government and that the toxic substance that led victims to choke was “likely chlorine.”

The use of chlorine as a weapon in Syria, if confirmed, would pose a dilemma for Washington, since it could not conceivably seek to rid Syria of a widely available chemical with numerous legitimate uses.

“There’s reluctance to call attention to it because there’s not much we can do about it,” a senior U.S. official said. “You can’t ask a country to get rid of all its chlorine.”

U.S. officials are also reluctant to provoke a confrontation over a lower-grade chemical until Syria’s stockpiles of sarin, a nerve agent, and other deadly materials have been removed, another U.S. official said.

The Syrian government has denied that it has used chlorine in weapons.

Daryl G. Kimball, the executive director of the Washington-based Arms Control Association, called the possible use of chlorine in attacks by the Syrian government “puzzling and disturbing.” When combined with munitions, he said, chlorine, a choking agent, makes them deadlier, but not nearly on the scale of sarin or mustard gas.

“This does not have a major effect on the strategic situation on the ground,” he said. “This is not the kind of weapon that Syria needs to use from a military perspective.”

Experts said that Assad’s alleged use of chlorine reflects a calculation by the regime that continued chemical strikes are valuable in intimidating rebel factions even if they are not as deadly as conventional munitions. Further, Assad seems confident that there will be no fallout.

“This says more about the determination of the regime to intimidate, kill, defeat the rebels,” said David Kay, a former U.N. weapons inspector now at the Potomac Institute for Policy Studies. “It says a lot about their lack of fear of consequences. What’s the West going to do? It’s done nothing so far.”

The OPCW says that the terms of the convention, which have applied to all signatories that destroyed stockpiles, requires Syria to eliminate facilities that were used to produce and store chemical weapons.

“Until and unless all of the declared material is removed from the country,” and any lingering questions about additional, undeclared stockpiles are addressed, “it is unwise for OPCW to be satisfied with leaving these production facilities partially intact,” Kimball said.

Michael Luhan, a spokesman for the organization, declined to comment Wednesday.

Mikulak, the U.S. envoy to the chemical weapons watchdog organization, said Syria has been unwilling to discuss the destruction of facilities that formed part of its chemical arsenal with other members of the convention.

“Twelve chemical weapons production facilities declared by Syria remain structurally intact,” he said. “Why is that? The answer is Syria’s intransigence.” He charged that “the Assad regime has delayed the operation at every opportunity.”

Mikulak said that Damascus has blamed delays for its failure to meet deadlines on the difficulty of accessing stockpiles in contested areas — an argument he dismissed.

U.S. officials said Russia, Syria’s principal arms supplier, continues to put pressure on Damascus to turn over the remaining sarin, making the effort one of the few remaining areas in which Moscow and Washington are collaborating.

“We need to see immediate and tangible signs that Syria intends to transport, in the very near future, the remaining chemicals from the site,” Mikulak said. “The international community cannot wait indefinitely for Syria action.”