The Britain-based Syrian says the tit-for-tat kidnappings in predominantly Sunni Muslim Idlib province could trigger sectarian clashes in the area.
The Observatory said the 42 Shiites, mainly women and children, were snatched Thursday from a bus that was traveling from the Shiite villages of Foua and Kfarya to the capital Damascus.
Observatory director Rami Abdul-Rahman said Saturday it was not clear who kidnapped the Shiites.
The U.N. Special Representative on Sexual Violence in Conflict, Zainab Hawa Bangura, called for the release of the women.
She said "allegations of abduction and rape of women and girls by armed groups have been received."