Christian Solidarity Worldwide (CSW) is calling for the release of 12 nuns and three young women who were taken from a convent in the Syrian town of Maaloula by Islamist rebels on Monday.
The majority of the town's Christian population fled when it was invaded by the Free Syrian Army (FSA) and Islamist militia, including Jabat al-Nusra which is linked with Al-Qaida, in September.
However, 40 nuns chose to stay in order to look after the town's many orphaned children.
CSW's Chief Executive Mervyn Thomas said in a statement, "Our thoughts and prayers are very much with the nuns and the three young women in their entourage".
He warned of a "deliberate targeting of clergy and laity by Islamist militia", noting that the whereabouts of two Archbishops abducted in April this year are still not known.
CSW has urged every party in Syria to "adhere to humanitarian standards with regard to the treatment of civilians, religious leaders and religious establishments, regardless of creed or ethnicity".
"We also request the speedy release of these nuns and their associates into the hands of church authorities," Mr Thomas finished.
The Pope has also voiced an appeal for the release of the nuns and called for increased prayer for the welfare of the women.
Speaking during a catechesis in Vatican City, he said: "Let us pray for these nuns, for these sisters and for all those who have been kidnapped as a result of the current conflict.
"Let us continue to pray and act for peace together."
The Syrian Observatory for Human Rights has estimated that the civil war has so far claimed at least 125,835 lives, with five million Syrians internally displaced and an estimated two million fleeing to other countries.