UNITED NATIONS (Reuters) - Investigators from the global chemical weapons watchdog found that two Syrian women were exposed to sulfur mustard in an apparent attack in Aleppo province last September, a report to the United Nations Security Council said.

The Organisation for the Prohibition of Chemical Weapons (OPCW) Fact Finding Mission was not able to visit the site. Investigators based their finding on interviews with the women, an analysis of blood samples taken under their supervision, and a review of information provided by the Syrian government and Russia.

“The FFM can confirm that the two female casualties reported to have been involved in the incident in Um Hosh, Aleppo of 16 September 2016 were exposed to sulfur mustard,” the mission wrote in a report to the Security Council earlier this month and made public on Tuesday.

A Russian team of investigators visited the site two months after the reported attack. The OPCW mission examined a mortar reported to be linked to the attack, which had been recovered by the Russian team and given to the Syrian government.

“Supported by the results of laboratory analysis, the FFM has determined that this mortar ... is a munition containing sulfur mustard,” the report said.

The OPCW Fact Finding Mission is only responsible for determining if chemical weapons were used in attacks in Syria.

A joint United Nations and OPCW investigation could now look at the incident to determine who is to blame. This team has already found Syrian government forces were responsible for three chlorine gas attacks in 2014 and 2015 and that Islamic State militants used mustard gas.

Syria agreed to destroy its chemical weapons in 2013 under a deal brokered by Russia and the United States.

The OPCW mission is also investigating a gas attack last month in Syria that killed dozens of people and prompted the United States to launch missiles on a Syrian air base. The OPCW has said a sarin or sarin-like substance was used.

Western powers blame Syrian government forces for the attack in the rebel-held area of northern Syria. The Syrian government, which is backed by Russia, has denied responsibility.

A crackdown by Syrian President Bashar al-Assad on pro-democracy protesters in 2011 led to civil war and Islamic State militants have used the chaos to seize territory in Syria and Iraq. Half of Syria’s 22 million people have been uprooted and more than 400,000 killed.