Doctors Without Borders said on Tuesday it had treated four patients showing symptoms of exposure to chemical agents, Newsweek reports.

The patients, an adult man and woman, a three-year-old girl and a five-day-old girl, were all from the same family, the charity said.

The family lived in Marea, Syria, a town about 45 kilometers (approximately 27 miles) north of Aleppo. Their house had been hit by a mortar shell on the evening of August 21. The family said that after the shell hit their house, yellow gas filled their home.

The family was first treated at a local hospital before being moved to the Doctors Without Borders facility, according to the Newsweek report.

The family experienced difficulty breathing, skin inflammation, red eyes and conjuctivitis. Several hours after seeking treatment, their conditions worsened as they developed blisters and their breathing became more difficult. As a result, the family was moved to a specialized treatment facility.

“MSF has no laboratory evidence to confirm the cause of these symptoms. However, the patients’ clinical symptoms, the way these symptoms changed over time, and the patients’ testimony about the circumstances of the poisoning all point to exposure to a chemical agent,” said Pablo Marco, the charity’s program manager for Syria, according to Newsweek.

“Any use of chemical weapons constitutes an extremely severe violation of international humanitarian law. It adds one more degree of suffering to a population who are already bearing the consequences of the worst humanitarian crisis in recent years. We appeal to all parties to observe a basic respect for human life and to stop indiscriminate violence against civilians,” he added.

It remains unclear exactly what the gas the family inhaled, though some reports indicate it might have been mustard gas. Another 50 patients suffering from similar symptoms to the family were treated by the Syrian American Medical Society, according to Newsweek. 

The statement from Doctors Without Borders follows recent reports that the Islamic State (ISIS) jihadist group has its hands on chemical weapons and had used them in attacks in Syria and Iraq.

Officials who spoke to CNN late last week said that the United States government has test results from an ISIS attack in Syria that confirm the terror group used a mustard agent as a weapon. 

On Friday, a senior American military official confirmed preliminary tests had showed traces of sulfur mustard on mortars fired by ISIS militants.

Brig. Gen. Kevin Killea, chief of staff for the military operations in Iraq and Syria, said the field testing into whether mustard gas was used is not conclusive, so final tests are underway to get the full make-up of the chemicals.

He did say, however, that the early tests showed the chemical agent on fragments from mortar rounds fired on August 11 against Kurdish Peshmerga forces in Iraq.

Information regarding the use of mustard gas by ISIS is relatively new. U.S. intelligence agencies have said in the past they believed the group has used chlorine gas in attacks in Iraq, though chlorine is not a banned chemical agent.