Signs reading "Toxic" are seen on the containers carrying Syria's dangerous chemical weapons aboard the Danish cargo ship, Ark Futura, transporting the chemical weapons out of Syria. (Petros Karadjias/AP)

ONE GRIM indication that the regime of Bashar al-Assad has been emboldened by the U.S. air campaign in Syria is the fresh reports of chemical weapons attacks on civilian areas. The Institute for the Study of War has compiled 18 allegations by Syrian sources of chlorine gas attacks by the regime since U.S. strikes against the Islamic State began in August. The first strike was reported Aug. 19 — the same day that the Organisation for the Prohibition of Chemical Weapons (OPCW) said it had completed the neutralization of the chemical weapons stockpile surrendered by the regime. The most recent was reported last week, when government forces allegedly used chlorine gas against rebel positions in the suburban Damascus area of Jobar.

How can the Assad regime still resort to chemical-weapons attacks months after the completion of a U.S.-led disarmament operation that President Obama claims as a success? One reason is that chlorine was not included in the chemicals the regime was obliged to hand over, even though its use in war violates the Chemical Weapons Convention, which Syria joined as part of the Russian-brokered deal. So while handing over stocks of sarin and mustard gas, the Assad forces have been fashioning “barrel bombs” containing chlorine and dropping them from helicopters on neighborhoods held by rebel forces.

After initial reports of such attacks last April, the OPCW dispatched a fact-finding team to investigate. Last month it reported “compelling confirmation” that toxic chemicals were used “systematically and repeatedly” as weapons in villages in northern Syria. The report noted that after the investigation began, “there was a marked reduction in reported chlorine attacks in the months of May, June and July.” But as world attention turned toward the U.S.-led air offensive against the Islamic State in August, the Assad barrel bombs began falling again. “In most cases,” the Institute for the Study of War reported, strikes “occurred in locations inaccessible to OPCW or Human Rights Watch investigators,” making verification virtually impossible.

What emerges from the various reports is that the Assad regime is once again blatantly violating the “red line” drawn by Mr. Obama against the use of chemical weapons — and getting away with it. Simon Limage, a State Department nonproliferation official, said this week that the “evidence strongly suggests the Assad regime is the culprit” for the Syria attacks. The Islamic State, too, may be using chlorine; The Post’s Loveday Morris reports a gas attack that occurred in Iraq, 50 miles north of Baghdad, last month.

The difference is that, while the United States has mobilized a coalition against the Islamic State, Mr. Assad is taking advantage of the fact that the U.S. strategy in Syria is to ignore him. Mr. Obama is resisting entreaties from the rebels and from the Turkish government to make the Assad forces a target, or even to declare a no-fly zone for the Syrian air force. So the regime calculates that chemical attacks that once prompted Mr. Obama to order retaliatory airstrikes now can be carried out with impunity.

Mr. Limage, who was in Brussels, told reporters that the United States was consulting with allies about a response to the chemical weapons attacks, according to the Wall Street Journal. But he added, “the least satisfying part of the conversation is next steps.” That will be true as long as Mr. Obama insists on giving the Assad regime a pass in his Syria campaign.