Syrian authorities have detained children in the past, usually to pressure family members to surrender.
The Nusra video showed the three children at the scene where the exchange took place. One little girl wept for her mother, and a masked rebel gunman held another child as he shouted "God is great!" in celebration.
Monday's exchange was reminiscent of a deal reached in October, when nine Lebanese pilgrims held in Syria for a year and a half were exchanged for two Turkish pilots held hostage in Lebanon. That deal, negotiated by Qatar and Palestinian officials, reportedly also included freeing dozens of women held in Syrian government jails to satisfy the rebels who abducted the pilgrims.
The nun's ordeal began in early December, when they were seized by rebels from a convent in the ancient Christian town of Maaloula that was overrun by rebels.
Upon their arrival to Damascus, the capital's residents gave the nuns a warm welcome at the Church of the Cross located in the predominantly Christian neighborhood of Qassaa, where prayers were held Monday evening marking their safe return.
Although the nuns appear to have been treated well, their seizure confirmed the fears of many Syrian Christians that they were being targeted by extremists among the rebels in the increasingly sectarian three-year conflict.
The country's chaotic mix of rebel groups is overwhelmingly Sunni Muslim, while minorities include Christians, Shiite Muslims and Alawites — whose sect is a Shiite offshoot. Most have sided with Assad or remained neutral, fearing for their fate should rebels take power. Assad is an Alawite.
Two bishops were seized in rebel-held areas in April, and an Italian Jesuit priest, Father Paolo Dall'Oglio, went missing in July after traveling to meet militants in Raqqa. None have been heard from since.
Also Monday, the international rights group Amnesty International accused the Syrian government of committing war crimes and crimes against humanity by blockading and starving civilians in the southern Damascus neighborhood of Yarmouk.
Amnesty said it estimated 128 people starved to death in Palestinian-dominated Yarmouk since a yearlong blockade on the area was tightened in July by forces loyal to Assad, who sought to flush out rebels and to punish civilians for harboring them.
Efforts to reach a truce in Yarmouk allowing food deliveries to starving residents have repeatedly collapsed.
AP reporters Diaa Hadid in Beirut and Adam Schreck in Dubai contributed reporting.
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