More than 100 chemical weapons sites are suspected to remain in Syria, left behind after the fall of the longtime president, Bashar al-Assad, according to the leading international organization that tracks these weapons.

That number is the first estimate of its kind as the group, the Organization for the Prohibition of Chemical Weapons, seeks to enter Syria to assess what remains of Mr. al-Assad’s notorious military program. The figure is far higher than any that Mr. al-Assad has ever acknowledged.

The sites are suspected to have been involved in the research, manufacturing and storage of chemical weapons. Mr. al-Assad used weapons like sarin and chlorine gas against rebel fighters and Syrian civilians during more than a decade of civil war.

The number of sites, and whether they are secured, has been a mystery since rebels toppled Mr. al-Assad last year. Now, the chemicals represent a major test for the caretaker government, which is led by the group Hayat Tahrir al-Sham. The group is designated as a terrorist organization by the United States, but it has renounced its links to Al Qaeda.

The stakes are high because of how deadly the weapons are, particularly when used in densely populated areas. Sarin, a nerve agent, can kill within minutes. Chlorine and mustard gas, weapons made infamous in World War I, burn the eyes and skin and fill the lungs with fluid, seemingly drowning people on land.