This image posted on a militant Web site in 2014 shows fighters from the Islamic State marching in Raqqa, Syria. (Militant Web site via AP)

Doctors and an aid organization said this week that the Islamic State appears to have used mustard gas during an attack last week in northern Syria, a claim that intensifies fears that the extremist group has obtained banned chemical weapons.

The incident Friday apparently involved dozens of mortar rounds that struck a village north of Aleppo called Marea, where the militant group is battling other armed opponents of the Syrian government. In video testimony, a doctor from the area said the Islamic State appears to have used mustard gas in the attack, which affected mostly “women and children.”

The Syrian American Medical Society said it treated 50 people who showed symptoms of exposure to chemical agents, including wheezing, skin irritation and severe itching. The humanitarian group, which operates a field clinic in the area, said 30 of those people developed severe blisters that doctors linked to mustard gas. The group said that “local sources” blamed the attack on the Islamic State, also known as ISIS and ISIL.

Evidence is mounting that the Islamic State has used mustard agent in recent attacks in Syria and Iraq, countries where the group controls vast territory. The access to and weaponization of such banned chemical agents would mark a serious escalation in the threat posed by the group, which is being targeted by a U.S.-led coalition.

U.S. military officials said recently that Islamic State militants used mustard gas during an attack this past month on Kurdish forces elsewhere in Syria. U.S. officials also have linked the group to an attack on Kurdish forces in Iraq that appeared to involve mustard gas.

Syria’s government has been accused of using chemical weapons over the course of the country’s four-year-old conflict, including a recent spate of attacks in the north involving chlorine gas, a choking agent. The West accuses President Bashar al-Assad’s forces of using sarin gas, a nerve agent, in an attack two years ago near the capital, Damascus, that killed hundreds.

In a statement released Tuesday, the humanitarian organization Doctors Without Borders said it treated four patients, all members of one family from Marea, who showed signs of exposure to chemical agents. The family’s home was struck Friday with a shell that emitted a yellow gas that filled the living room, the group said. Although it did not indicate whether mustard gas was used, the group said the family members’ symptoms “all point to exposure to a chemical agent.”

The relatives arrived at a Doctors Without Borders hospital in the area an hour after the attack and were “suffering from respiratory difficulties, inflamed skin, red eyes and conjunctivitis. Within three hours they developed blisters and their respiratory difficulties worsened,” the group said.

It is unclear how the Islamic State would have gained access to mustard gas, which Iraq’s former leader, Saddam Hussein, had stockpiled and used on his own people.

The Assad government said this past year that its chemical weapons stockpiles, including mustard gas, had been destroyed under an agreement brokered by the United States and Russia. But that assertion has come under question by weapons inspectors and U.S. intelligence officials, who suspect that the Syrian leader has concealed some of the cache.

Meanwhile, U.N. Secretary General Ban Ki-moon has accused all sides in the conflict of “indiscriminate and disproportionate attacks” on civilians and said the United Nations and its partners could not deliver any food to 466,000 people in besieged areas last month, the Associated Press reported.

In a monthly report to the U.N. Security Council circulated Tuesday, he said access to the 4.6 million Syrians in hard-to-reach areas — most controlled by the Islamic State — also remains extremely limited.

Read more:

Assad’s regime is at increasing risk amid surge of rebel attacks

Everything you need to know about Syria’s chemical weapons

U.S. allies in Middle East ramping up support for rebels in Syria