The United Nations has concluded that Syrian President Bashar Assad used chemical weapons on civilians during Syria's devastating, five-year civil war.

The determination was made after a year-long investigation and was released the same day reports surfaced that Assad may still have some chemical weapons.

The investigation, done in conjunction with the Organization for the Prohibition of Chemical Weapons, found that Assad has repeatedly used chlorine as a weapon. Investigators also confirmed that the self-proclaimed Islamic State used mustard gas on Syrians last year.

"The U.S.-led Counter-ISIL coalition has placed a high priority on targeting ISIL's chemical weapons capabilities, including by capturing one of its chemical weapons manufacturing leaders in March 2016 and using information gained from him to launch airstrikes to degrade ISIL's ability to use such weapons," National Security Council spokesman Ned Price stated on Wednesday. "We continue to remove leaders from the battlefield with knowledge of these weapons and will target any related materials and attempts to manufacture such chemicals going forward."

Assad's use of chlorine as a weapon violates the Chemical Weapons Convention and a U.N. Security Council resolution.

"The United States will work with our international partners to seek accountability through appropriate diplomatic mechanisms," Price stated. "We urge all U.N. member states and parties to the Chemical Weapons Convention, including Russia and Iran, to participate in this effort."