WASHINGTON (Reuters) - The United States on Thursday blacklisted 18 senior Syrian officials it said were connected to the country’s weapons of mass destruction program, after an international investigation found Syrian government forces were responsible for chlorine gas attacks against civilians.

The action marked the first time the United States has sanctioned Syrian military officials for the government’s use of chemical weapons, according to a Treasury Department statement.

A joint inquiry by the United Nations and the Organization for the Prohibition of Chemical Weapons (OPCW) found that Syrian government forces were responsible for three chlorine gas attacks and that Islamic State militants had used mustard gas, according to reports seen by Reuters in August and October.

Chlorine’s use as a weapon is banned under the Chemical Weapons Convention, which Syria joined in 2013. If inhaled, chlorine gas turns into hydrochloric acid in the lungs and can kill by burning lungs and drowning victims in the resulting body fluids.

Syrian President Bashar al-Assad’s government has denied its forces have used chemical weapons.

“We condemn in the strongest possible terms the Syrian regime’s use of chemical weapons,” Ned Price, a White House National Security Council spokesman, said in a statement. “The Assad regime’s barbaric continued attacks demonstrate its willingness to defy basic standards of human decency, its international obligations, and longstanding global norms.”

Following the reports of the international inquiry, Britain and France circulated a draft resolution to the U.N. Security Council in December that would ban the sale or supply of helicopters to the Syrian government and blacklist 11 Syrian military commanders and officials over chemical weapons attacks during the nearly six-year war.

A vote on the draft resolution has not yet been set, but diplomats said Syrian ally Russia, one of five council veto powers, has made clear it opposed the measures.

Ten of the individuals sanctioned by the United States on Thursday are listed for designation in the draft resolution, which – if adopted - would subject them to a global travel ban and asset freeze.

Russian U.N. Ambassador Vitaly Churkin said in November that there was “just not enough material proof to do anything” and described the French and British bid to impose U.N. sanctions as a “misplaced effort.”

Syria agreed to destroy its chemical weapons in 2013 under a deal brokered by Moscow and Washington. The Security Council backed that deal with a resolution that said in the event of non-compliance, “including unauthorized transfer of chemical weapons, or any use of chemical weapons by anyone” in Syria, it would impose measures that could include sanctions.