Members of the Islamic State of Iraq and the Levant kidnapped father Paolo Dall'Oglio while he was walking in Raqqa, which fell to militant Islamist brigades in March, the sources in Raqqa province told Reuters.
Syrian authorities expelled Dall'Oglio from the country last year for helping victims of Assad's military crackdown while he served at a sixth-century monastery in the Anti-Lebanon mountains north of Damascus.
He has been an advocate of reconciliation for the country's myriad religious and ethnic sects, especially between Kurds and Arabs, as Syria slipped into civil war. Dall'Oglio blamed Assad for provoking sectarian mayhem and called his forces "thugs."
Abdelrazzaq Shlas, a leading opposition activist in Raqqa, said the Islamic State appeared to have been angered by comments Dall'Oglio had made criticising violence against Kurdish residents of Tel al-Abiad, on the border with Turkey.
Fighting flared in the town and other areas to the northeast in recent weeks between Islamist militants and members of the Kurdish Workers Party (PKK) militia.
"Father Paolo was received in Raqqa with open arms but the Islamic State appears not to have taken well to his position regarding Tel al-Abiad," Shlas said.
Shlas pointed to crowds in Raqqa cheering Dall'Oglio during a street rally he attended on Sunday night in support of the besieged city of Homs. Video footage of the rally showed Dall'Oglio saluting the crowd.
A Western diplomat said Dall'Oglio crossed into Syria from Turkey last week, ignoring warnings from his friends not to go to Raqqa, where Islamist militants kidnapped several liberal activists in recent weeks.
"He insisted on going," the diplomat said.
Dall'Oglio served for three decades at the Monastery of Saint Moses the Abyssinian, or Deir Mar Musa, before being expelled from the country in 2012. He was instrumental in restoring the monastery, whose cathedral houses an exquisite 11th century fresco of the Last Judgment.
In a statement from Raqqa published on Facebook before his disappearance, Dall'Oglio said he considered Syria to be his homeland.
"I feel happy because first I am in a liberated city and second because of the marvelous reception I have gotten. People walk the streets with freedom and in harmony. It is a picture of the homeland we want for all Syrians," Dall'Oglio said.