Fr Diyaa Aziz, 40, is one of six priests in total known to have been captured in Syria since 2012. The priest is from the predominantly Christian village of Jacoobiya, in the north western province of Idlib.
Aid to the Church in the Need has said the Al-Nusra Front extremist group asked Fr Aziz to meet with its leader, and that when the priest obliged, he was never seen again.
The advocacy group has said the other captured priests are: Archbishops Boulos Yazigi and Youhanna Ibrahim of Aleppo; Fr Paolo Dall'Oglio; Fr Hanna Jallouf, and; Father Jacques Mourad.
The whereabouts and state of health of all the prisoners, including Fr Aziz, are currently unknown.
Two others priests have been killed since 2012: Fr Frans van der Lugt and Fr Hadi Haddad. Fr Lugt was caught in fighting between government and rebel fighters and
Neville Kyrke-Smith, National Director of Aid to the Church in Need UK, said: "We plead for prayers and mercy - for young Fr Aziz, for Fr Jacques Mourad and all the priests and bishops who have been kidnapped and for their people.
"It is as though Daesh (IS) and other fanatical groups want to cut off the heads of the Christian communities to kill the body - the Church - and they should know that this will not work."
"Having been in northern Iraq recently and knowing the terror that Christian communities and others face in great parts of Syria and Iraq, the people need the support, solidarity and resolve of the West to stand up to this contagion of evil."
"If anybody wishes to rebuild the Middle East we need the leaders of different communities to be respected and allowed to work with all people."