A young man sits on a bed on March 17, 2015 at a clinic in the village of Sarmin, southeast of Idlib, the capital of Syria’s northwestern province of Idlib, following reports of suffocation cases related to an alleged regime gas attack in the area.

The United States is deeply disturbed by reports that Syrian government forces attacked the town of Sarmin using chlorine as a weapon on Tuesday, Secretary of State John Kerry said in a statement on Thursday.

“We are looking very closely into this matter and considering next steps,” he said. “While we cannot yet confirm details, if true, this would be only the latest tragic example of the Assad regime’s atrocities against the Syrian people, which the entire international community must condemn.”

A group monitoring the Syrian civil war said on Tuesday that government forces carried out a poison gas attack that killed six people in the northwest, and medics have posted videos of children suffering what they said was suffocation.

But a Syrian military source described the report of an attack on the town in Idlib province as propaganda.

Kerry has been getting tough on Syria in recent days after raising concerns among Middle East allies that the United States is open to negotiating with Assad, who has been fighting Islamist and other rebels since 2011. The State Department has said the United States is seeking a negotiated political settlement but one that excludes Assad.

Syria agreed in 2013 to destroy its entire chemical weapons program under a deal brokered with the United States and Russia after hundreds of people were killed in a sarin gas attack on the outskirts of the capital, Damascus.

Syria’s ambassador to the global chemical weapons watchdog said on Thursday that his country condemned any use of chemical weapons, without mentioning any specific incidents, according to Syrian state news agency SANA.

Envoy Basam al-Sabagh said Damascus had provided the Hague-based Organisation for the Prohibition of Chemical Weapons (OPCW) with documents in December showing armed groups had used chlorine gas in Syria, the SANA report said.

It cited Sabagh’s comments to the OPCW’s executive council in a session on Thursday in which he also said Syria had fulfilled all of its obligations under the Chemical Weapons Convention.

The OPCW has found evidence that chlorine gas was repeatedly used as a weapon in the country.

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