U.S. Secretary of State John Kerry said Thursday there is evidence that Syrian President Bashar Al-Assad is using chlorine gas, “in violation” of a global chemical weapons treaty.

"We believe there is evidence of Assad's use of chlorine," Kerry testified before the House Foreign Affairs Committee, according to The Hill.

Chlorine gas is not on the list of chemicals Syria declared to the international community, but Kerry said its use is prohibited by the Chemical Weapons Convention.

The secretary said the U.S. also had “some questions” about Assad honoring other parts of the weapons treaty.

Last week, the Organization for the Prohibition of Chemical Weapons(OPCW) confirmed that chlorine gas was used in attacks in northern Syria this year.

The OPCW, which is the international body seeking to implement global laws banning chemical weapons, said it had found "compelling confirmation" of the use of chlorine, in pure or mixed form.

The attacks took place earlier this year in villages where rebels have been fighting government forces, according to the organization, which has been overseeing efforts to destroy Syria's chemical weapons.

Activists in Syria said several months ago that over 100 people had been killed in a chlorine gas attack in the town of Talmenes in Idlib province, and the United Nations Security Council has called for an investigation into the alleged attack. Since then there were reports of other such attacks.

Kerry himself said a few months ago that he has seen “raw data” indicating that the Syrian government has used chlorine gas as a chemical weapon in a “number of instances” in recent months.

Syria has emphatically denied that it carried out chlorine gas attacks against civilians.

Kerry’s testimony came a day after the House voted to arm moderate Syrian rebels to combat the Islamic State in Iraq and Syria.

He pushed back on any suggestions that the U.S. views the Syrian regime as legitimate, saying there is no "long-term future" for Assad in power.

"The Syrian opposition is not going to stop fighting Assad. We recognize that reality," Kerry said, according to The Hill.