BEIRUT (Reuters) - Chemicals dropped from the air caused at least nine people to suffer breathing problems in an attack in northwest Syria, rescue workers and doctors said on Monday.

The Syrian American Medical Society (SAMS), a charity which supports hospitals in Syria, said its doctors in Idlib reported 11 patients “with symptoms indicative to usage of chlorine”, SAMS advocacy manager Mohamad Katoub said on his Twitter page.

Radi Saad, from the chemical weapons team of the White Helmets civil defense group that operates in rebel-held parts of Syria, told Reuters three of the nine people who suffered from “suffocation injuries” were rescuers responding to the incident.

Two barrels containing chemical gasses had been dropped from helicopters on Sunday night, Saad said.

The Syrian government has consistently denied using chlorine or other chemical weapons during Syria’s conflict, now approaching its eighth year.

Air raids intensified on rebel-held towns and cities in northwest Syria’s Idlib province on Sunday night, a day after rebels shot down a Russian warplane and killed its pilot.

Syrian President Bashar al-Assad, with the help of Russian air power and Iran-backed militias, has said he wants to take back control off all of Syria. Syrian government and allied forces have advanced into rebel-held areas of northwest Syria in recent weeks.

The Britain-based Syrian Observatory for Human Rights said a number of cases of suffocation were reported after helicopters targeted the town of Saraqeb on Sunday.

Rescue workers and medical groups have also accused government forces of using chlorine gas against the rebel-held Eastern Ghouta district near the capital Damascus three times over the last month, most recently on Thursday.

Syria agreed to give up its chemical weapons arsenal in 2013. In the past two years, a joint inquiry by the United Nations and the Organization for the Prohibition of Chemical Weapons (OPCW) has found the Syrian government used the nerve agent sarin and has also several times used chlorine as a weapon. The inquiry also said the Islamic State group has used sulfur mustard.

The German government called on Monday for a thorough investigation into reports Syria had used chemical weapons in both Idlib and Eastern Ghouta.

“If it is confirmed that the Syrian government has once again used chemical weapons, that would be an abhorrent act and an egregious violation of the moral and legal obligation to avoid the use of chemical weapons,” an official with the German foreign ministry said.

U.S. Defense Secretary Jim Mattis said last week that the Syrian government had repeatedly used chlorine as a weapon, and Washington was also concerned about the potential use of sarin gas.